shader_cache: Implement a generic shader cache

Implement a generic shader cache for fast lookups and invalidations.
Invalidations are cheap but expensive when a shader is invalidated.

Use two mutexes instead of one to avoid locking invalidations for
lookups and vice versa. When a shader has to be removed, lookups are
locked as expected.

src/video_core/CMakeLists.txt

@@ -93,6 +93,7 @@ add_library(video_core STATIC
     renderer_opengl/utils.h
     sampler_cache.cpp
     sampler_cache.h
+    shader_cache.h
     shader/decode/arithmetic.cpp
     shader/decode/arithmetic_immediate.cpp
     shader/decode/bfe.cpp