st/egl: Implement swapbuffer throttling

st/egl: Implement swapbuffer throttling

When doing copy swapbuffers using drm, throttle on outstanding copy operations.
Introduces a new environment variable, EGL_THROTTLE_FENCES that the
user can use to indicate the desired number of outstanding swapbuffers, or
disable throttling using EGL_THROTTLE_FENCES=0.

This can and perhaps should be extended to the pageflip case as well, since
with some hardware pageflips can be pipelined. In case the pageflip syncs, the
throttle operation will be a no-op anyway.

Update copyright notices.

Signed-off-by: Thomas Hellstrom <thellstrom@vmware.com>