Validate varying count when compiling shaders

This avoids crashing on devices when a number of varyings greater than the device limit is used.

For now this accurately prints an error when compiling the shader, but the error text only pops up in the editor if the number of user varyings is above the limit.

servers/rendering/dummy/storage/utilities.h

@@ -94,6 +94,8 @@ public:
 	virtual String get_video_adapter_api_version() const override { return String(); }
 
 	virtual Size2i get_maximum_viewport_size() const override { return Size2i(); }
+	virtual uint32_t get_maximum_shader_varyings() const override { return 31; } // Fair assumption for everything except old OpenGL-only phones.
+	virtual uint64_t get_maximum_uniform_buffer_size() const override { return 65536; } // Fair assumption for all devices.
 };
 
 } // namespace RendererDummy