GPU: Enable GL multithreaded compilation by default

This allows to reduce the waiting time caused by
shader compilation on some GPU-driver combo.

A new settings in the User Preferences make it
possible to override the default amount of worker
threads and optionally use subprocesses.

We still use only one worker thread in cases where
there is no benefit with adding more workers
(like AMD pro driver and Intel windows).

It doesn't scale as much as subprocesses for material
shader compilation but that is for other reasons
explained in #139818.

Add some heuristic to avoid too much memory usage
and / or too many stalls.

Also add some heuristic to the default number of subprocess for
the platform that shows scalling.

Historically, multithreaded compilation was prevented by the
need of context per thread inside `DRWShader` module.
Also there was no good scaling at that time. But
nowadays numbers shows different results with
good scaling with reasonable amount of threads on many
platforms.

Even if we are going for vulkan in the next release
most of the legacy hardware will still use OpenGL for
a few other releases. So it is relevant to make this
easy improvement.

See pull request for measurements.

Pull Request: https://projects.blender.org/blender/blender/pulls/139821

release/datafiles/userdef/userdef_default.c

@@ -116,7 +116,8 @@ const UserDef U_default = {
 #else
     .gpu_backend = GPU_BACKEND_OPENGL,
 #endif
-    .max_shader_compilation_subprocesses = 0,
+    .gpu_shader_workers = 0,
+    .shader_compilation_method = USER_SHADER_COMPILE_THREAD,
 
     /** Initialized by: #BKE_studiolight_default. */
     .light_param = {{0}},