set the blend color
|red, green, blue, alpha||
specify the components of GL_BLEND_COLOR
The GL_BLEND_COLOR may be used to calculate the source and destination blending factors. The color components are clamped to the range before being stored. See glBlendFunc for a complete description of the blending operations. Initially the GL_BLEND_COLOR is set to (0, 0, 0, 0).
glGet with an argument of GL_BLEND_COLOR
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