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.

drivers/vulkan/rendering_device_driver_vulkan.cpp

@@ -5886,6 +5886,10 @@ uint64_t RenderingDeviceDriverVulkan::limit_get(Limit p_limit) {
 			return vrs_capabilities.max_fragment_size.x;
 		case LIMIT_VRS_MAX_FRAGMENT_HEIGHT:
 			return vrs_capabilities.max_fragment_size.y;
+		case LIMIT_MAX_SHADER_VARYINGS:
+			// The Vulkan spec states that built in varyings like gl_FragCoord should count against this, but in
+			// practice, that doesn't seem to be the case. The validation layers don't even complain.
+			return MIN(limits.maxVertexOutputComponents / 4, limits.maxFragmentInputComponents / 4);
 		default:
 			ERR_FAIL_V(0);
 	}