rasterizer_cache: Reintroduce method for flushing.

src/video_core/renderer_opengl/gl_buffer_cache.h

@@ -24,6 +24,9 @@ struct CachedBufferEntry final {
         return size;
     }
 
+    // We do not have to flush this cache as things in it are never modified by us.
+    void Flush() {}
+
     VAddr addr;
     std::size_t size;
     GLintptr offset;

src/video_core/renderer_opengl/gl_shader_cache.h

@@ -33,6 +33,9 @@ public:
         return GLShader::MAX_PROGRAM_CODE_LENGTH * sizeof(u64);
     }
 
+    // We do not have to flush this cache as things in it are never modified by us.
+    void Flush() {}
+
     /// Gets the shader entries for the shader
     const GLShader::ShaderEntries& GetShaderEntries() const {
         return entries;