Tested against Mesa master@8c78fdb with `ninja check-func` on Linux for
x11_egl, wayland, and gbm. However, the new codepaths were not tested
because Mesa does not yet support the needed eglGetPlatformDisplay
extensions.

v2:
  - For x11_egl, pass a `Display*` to eglGetPlatformDisplay, not a
    `Display**`. (for emil)
  - Rebase on other v2 patches.

Reviewed-by: Emil Velikov <emil.l.velikov@gmail.com>