B.Map compatibility

This change moves the "pivot" of rotation and scale operations to be at the center of the texture space (0.5, 0.5) rather than at the origin(0.0, 0.0). This matchs PixMap's transform behavior to be the same as B.Map!

A note on texture-space conventions:

After Effects, Metal, DirectX 12, Vulkan, and other graphics-library code put their texture-space origin (where (0.0, 0.0) is) at the top-left corner of the image.

OpenGL on the other hand is an exception to this convention and uses the bottom-left of of the image as the origin. PixMap and B.Map uses the top-left convention rather than making any exceptional assumptions about the incoming UVs. This will mean that if you have UVs that come from the OpenGL convention, your Offset and Tile-Y parameter might have to be adjusted to correct the incoming Y-coordinate.

Usually, just using a negative operand for the Tile-Y parameter should be enough. Depending on your V-Repeat usage, you may also need an additional Offset of (0.0, 1.0)