shader: Properly scale image reads and add GL SPIR-V support

Thanks for everything!

src/shader_recompiler/backend/bindings.h

@@ -14,6 +14,8 @@ struct Bindings {
     u32 storage_buffer{};
     u32 texture{};
     u32 image{};
+    u32 texture_scaling_index{};
+    u32 image_scaling_index{};
 };
 
 } // namespace Shader::Backend