compositor: Fix rendering with shm transformed buffers

compositor: Fix rendering with shm transformed buffers

The implementation of buffer transformation didn't handle transformed
shm buffers properly. The partial texture upload was broken since the
damage is in surface coordinates that don't necessarily match the
buffer's coordinates. It also wouldn't handle the buffer stride
properly, resulting in incorrect rendering if it didn't match the
buffer's width.

The logic used for converting texture coordinates was generalized and
moved out of the renderer, since this conversion may be useful in other
places, such as the backends.