egl: fence-fd vs. EGL_KHR_mutable_render_buffer fixes
The main bit is the last patch, which fixes an issue with
EGL_KHR_mutable_render_buffer implementation, in particular it is not valid to ask for an fd from a fence not created with the
PIPE_FLUSH_FENCE_FD flag. The first two patches are a small theoretical fix plus some debugging I cooked up while tracking down where the invalid fence fd was coming from.
Fixes a crash with the low-latency-stylus demo app.