Finally fixed layers sizes ?

- Added a type AvatarLayer that contains the TextureFrameBuffer and
scales along X and Y accounting for the switch from real sizes to
(always larger) integer pixels sizes.
- Updated LayerDP in UIAvatar for the more precise « at-pixel » sizes.
- UIListItem now account for the new sizing method when preparing to
render in a layer.
- Faster multisample image shaders ? (avoid to use repetitive mix)
- TextureFramebuffer has a « isMultiSampled() » method.
- Added a None color constant to Rgba representing fully transparent
black.