GPU Compositor: Avoid global DST lock on Linux

It is not required to hold the lock of DST when performing
compositing on GPU, as the compositor implementation uses the
GPU module directly, bypassing the draw manager.

However, currently this is known to cause issues on macOS,
and is not yet tested on Windows.

On Linux it works correctly, and avoids lock while compositor
is running.

There could still be a small locking hiccup, when the GPU
context is created and disposed. This needs to be looked
into.

@@ -520,0 +540,4 @@
+
+#if defined(__linux__)
+    GPU_flush();
+    GPU_render_end();