DRW: Shader Deferred compilation: Use a wmJob for threading.

Also get rid of the static var and initialization.
This enables the user to see the progress on the info header.
Closing blender or reading a file also kill the job which is good.

Unfortunatly, this job cannot be interrupt by users directly. We could make it interruptible but we need a way to resume the compilation.

source/blender/windowmanager/WM_api.h

@@ -504,6 +504,7 @@ enum {
 	WM_JOB_TYPE_POINTCACHE,
 	WM_JOB_TYPE_DPAINT_BAKE,
 	WM_JOB_TYPE_ALEMBIC,
+	WM_JOB_TYPE_SHADER_COMPILATION,
 	/* add as needed, screencast, seq proxy build
 	 * if having hard coded values is a problem */
 };