ProgramShaderCache: Create a FBO for shared contexts to draw into

This way it allows us to use surfaceless contexts in EGL/GLX. It also
ensures that the shared context shares a similar setup to the main
context's framebuffer, potentially reducing the number of variants a
driver needs to generate.

Source/Core/VideoBackends/OGL/ProgramShaderCache.h

@@ -154,6 +154,9 @@ private:
   struct SharedContextData
   {
     std::unique_ptr<cInterfaceBase> context;
+    GLuint prerender_FBO;
+    GLuint prerender_FBO_tex;
+    GLuint prerender_FBO_depth;
     GLuint prerender_VBO;
     GLuint prerender_VAO;
     GLuint prerender_IBO;