Add a few RGB-aware set & blend methods to the blitters

[smasher816]

Add new BB_fill_rect_color and BB_blend_rect_color functions taking a ColorRGB32. Use a new is_rgb flag in Color structs for lua wrapper to decide which BB_ function to call. If a ColorRGB32 is passed to fillRect/lightenRect then the new path will be taken. Otherwise original Color8A/uint8_t code is called.

No changes should occur for existing code paths. Callers must be patched to provide RGB colors if desired. Lua blitbuffer implementation has not been changed.

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This change is