Math for drawing font over byte buffer was incorrect. Effect can be seen
when target buffer is fully black and transparent - this results in font
color being effectively premultiplied, which causes problems when image
is composited further.
Use `blend_color_mix_byte()` and `blend_color_mix_float()` for blending.
Reviewed By: sergey
Differential Revision: https://developer.blender.org/D11035
6944521d7e