TODO

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Known bugs
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- outofmemory in arrays silently ignored
- blending doesn't use premultiplied colors (not porter-duff model)
- gradient color ramp is not interpolated in linear space
- linear and non-linear image formats behave both linearly
- radial gradient circle has got a fixed step instead of
  proportional to its radius

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Incomplete functionality:
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- non-zero fill mode
- proper porter-duff blending
- masking
- scissoring
- child images
- VG_DRAW_IMAGE_STENCIL
- _PRE and BW_1 image formats
- VGU warp matrices
- EGL (image drawing targets)
- boundbox / point-on-path / tangent-on-path queries
- RENDERING_QUALITY_FASTER == RENDERING_QUALITY_BETTER

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Possible portability issues
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- GL_UNSIGNED_INT_8_8_8_8 and other for images
- GL_MIRRORED_REPEAT for gradients
- GL_CLAMP_TO_EDGE for gradients and pattern paints
- GL_CLAMP_TO_BORDER for pattern paints
- multitexturing for VG_DRAW_IMAGE_MULTIPLY, VG_DRAW_IMAGE_STENCIL

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Bonus:
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- gradient shaders? antialiasing shader?