Various GL perf improvements

commit ba61b5b2:

gstgl: Don't do framebuffer checks in low debug levels
Framebuffer checks can be very expensive, taking up to 3-5% of the
total CPU consumed by the application.

commit acbc8549:

gstgl: Disable timing queries for GL memory functions when debugging
This can be very expensive in some workloads, taking up to 11% of the
total execution time.

commit 192ec168:

gstgl: Don't use condition variables for message synchronization
Using a single condition variable for synchronization across all GL
messages is very slow on Windows and uses up to 20% CPU usage in some
workloads due to lock contention and false broadcasts.

Using per-message event handles reduces the CPU usage to negligible
amounts despite having to allocate a new event handle for each
message.