render/egl: stop including eglmesaext.h
This is a Mesa-specific header that was needed because some Wayland EGL extensions were missing from the Khronos registry. Now that this has been fixed 1 and Mesa 2 & glvnd 3 have sync'ed their headers, we can drop this workaround.
- glvnd 1.3.2 contains the patch
- Mesa 20.2.0 contains the patch