Screen('ColorRange'): Add workaround for color clamping OpenGL driver…

… bugs.

Turns out the recent proprietary AMD OpenGL driver on MS-Windows has a broken
color clamping query implementation, which does not report clamp state for
GL_CLAMP_VERTEX_COLOR_ARB or GL_CLAMP_FRAGMENT_COLOR_ARB.

As such, we get reported failure to change color clamping on Windows 10 + AMD,
whereas the same code works fine on Windows 10 NVidia or Intel.

Work around this by detecting the failure and auto-selecting our own internal
GLSL shader based fallback path. This should fix it - at a performance penalty,
as vertex color clamping can be handled by our fallback shader, fragment color
clamping has proper default behaviour of clamping on for fixed point unorm
framebuffers, and clamping off for floating point framebuffers. Only readback
clamping needs to be controlled via glClampColorARB, but luckily readback
clamping is supported by glClampColor() as well, in a backwards compatible way,
so we should be good. Famous last words...

Note that this bug is so far only present on AMD on Windows, so we can get
away with only rebuilding Screen() for MS-Windows for the moment.

Also note that because it is the query that is broken, not the setting, our response
of selecting the fallback path may be not necessary. However, it is unlikely to hurt,
and we can not know, so better safe than sorry.

PsychSourceGL/Source/Common/Screen/SCREENColorRange.c

@@ -111,7 +111,7 @@ PsychError SCREENColorRange(void)
 	PsychWindowRecordType *windowRecord;
 	double maxvalue, clampcolors, oldclampcolors;
 	int applyToMakeTexture;
-	GLboolean enabled, enabled1, enabled2, enabled3;
+	GLboolean enabled, enabled1, enabled2, enabled3, clampDriverFail;
 	GLuint tunnelShader = 0;
 
 	//all subfunctions should have these two lines.  
@@ -152,35 +152,53 @@ PsychError SCREENColorRange(void)
 	if (oldclampcolors != clampcolors) {
 		// Yes. Try to set new clamping behaviour:
 		PsychSetGLContext(windowRecord);
+		clampDriverFail = FALSE;
 
 		// Does graphics hardware/OS support clamping mode change via glClampColorARB and shall
 		// we use it? A clampcolors setting of -1 would use our own shader based implementation
 		// even if the hardware could do it -- This to test the precision of our approach vs.
 		// hardware and to guarantee consistent results even if it means a performance hit.
 		if (glClampColorARB && (clampcolors>=0)) {
 			// Color clamping extension supported: Set new clamp mode in hardware via extension:
-
+			while (glGetError());
+
 			enabled = (clampcolors > 0) ? GL_TRUE : GL_FALSE;
+			enabled1 = enabled2 = enabled3 = !enabled;
+
 			glClampColorARB(GL_CLAMP_VERTEX_COLOR_ARB, enabled);
 			glClampColorARB(GL_CLAMP_FRAGMENT_COLOR_ARB, enabled);
 			glClampColorARB(GL_CLAMP_READ_COLOR_ARB, enabled);
 
+			if ((glGetError() == GL_INVALID_ENUM) && (PsychPrefStateGet_Verbosity() > 1))
+				printf("PTB-WARNING: glClampColorARB() failed as invalid enum unsupported.\n");
+
 			// Check if the clamp en-/disable worked:
 			glGetBooleanv(GL_CLAMP_VERTEX_COLOR_ARB, &enabled1);
 			glGetBooleanv(GL_CLAMP_FRAGMENT_COLOR_ARB, &enabled2);
 			glGetBooleanv(GL_CLAMP_READ_COLOR_ARB, &enabled3);
-
+
+			if ((glGetError() == GL_INVALID_ENUM) && (PsychPrefStateGet_Verbosity() > 1))
+				printf("PTB-WARNING: One of the clamp color queries failed as unsupported. Result may be unreliable, response may be a false positive.\n");
+
 			if ((clampcolors==0 && (enabled1 || enabled2 || enabled3)) || (clampcolors>0 && (!enabled1 || !enabled2 || !enabled3))) {
-				if (PsychPrefStateGet_Verbosity()>1) printf("PTB-WARNING: Could not %s color value clamping as requested. Unsupported by your graphics hardware?\n", (clampcolors>0) ? "enable" : "disable");
+				if (PsychPrefStateGet_Verbosity() > 1) printf("PTB-WARNING: Could not %s hardware color clamping as requested. Unsupported by your graphics hardware? Trying fallback.\n", (clampcolors>0) ? "enable" : "disable");
+				if (PsychPrefStateGet_Verbosity() > 3)
+					printf("PTB-DEBUG: %sClamp vertex %i, fragment %i, readback %i.\n", glClampColorARB == glClampColor ? "glClampColorARB() == glClampColor()! " : "", enabled1, enabled2, enabled3);
 
-				// Reset to old setting if the switch didn't work:
-				clampcolors = oldclampcolors;
+				// Mark failure, to trigger shader fallback:
+				clampDriverFail = TRUE;
 			}
 			else if ((PsychPrefStateGet_Verbosity() > 3) && (clampcolors==0)) {
 				printf("PTB-INFO: Disabled color clamping via hardware.\n");
 			}
+
+			// Eat OpenGL errors:
+			while (glGetError());
 		}
-		else {
+
+		// Use shader method if requested by user script, or if hw method unsupported, or if
+		// hw method failed due to OpenGL driver bug, e.g., as observed with AMD on MS-Windows:
+		if (!(glClampColorARB && (clampcolors>=0)) || clampDriverFail) {
 			// Color clamping extensions unsupported, or user wants our own implementation: We need to use quite a bit of shader and cpu magic...
 			if ((PsychPrefStateGet_Verbosity() > 3) && (clampcolors>=0)) printf("PTB-INFO: Switching and query of color clamping via glClampColorARB unsupported by your graphics hardware or operating system.\n");
 			if ((PsychPrefStateGet_Verbosity() > 3) && (clampcolors< 0)) printf("PTB-INFO: Switching of color clamping via internal shader-based solution forcefully enabled by usercode.\n");