Color and PixelAccessor updates and fixes.
Color:
- Constructor Color4ub( Color4f ) is explicit to prevent accidental information loss.
- Constructor Color4ub( uint32_t ) is explicit to prevent accidental conversions.
- Constructor Color4f( vector<float> ) is explicit.
PixelAccessor:
- writeColor(x,y,float) --> writeSingleValueFloat
- writeColor(x,y,uint8_t) removed (unused).
- writeSingleValueFloat(...) fixed for float textures.