Add STBIR_TYPE_UINT16_SRGB and STBIR_TYPE_FLOAT_SRGB

[p0nce]

Technically the rgba16 images in Gamut are still sRGB, but there is no way to resize being gamma aware using stb_image_resize[2].h, which always assume 16-bit images are linear.

• Update stbir__type_size
• "if we're completely point sampling, then we can turn off SRGB"
• encode_alphas and encode_simple
• decode_alphas and decode_simple

[p0nce]

Though, in pure honesty there is a range of power curve images could have