Add a mechanism to avoid automatically updating the shadow texture when using TFP on V3D

This MR contains the first three patches of MR !11804 (closed) so it only implements the gallium and GLX part. This is simpler to merge because it doesn’t require adding a new EGL extension.

The three patches have been slightly modified to add a rectangles parameter to the gallium interface so that a future version of the EGL extension could use it describe the region that has been modified. The V3D implementation currently ignores this parameter but it seems better to add it now so that we don’t have to change any future implementations if we want to add it later.