Fix build error in SDL_render_metal.m (bug #4916.)

Patch provided by Konrad. Error was introduced by commit 35f99d918ca2 for bug #4914.
diff --git a/src/render/metal/SDL_render_metal.m b/src/render/metal/SDL_render_metal.m
index f8ab1fc..6c43dd4 100644
--- a/src/render/metal/SDL_render_metal.m
+++ b/src/render/metal/SDL_render_metal.m
@@ -949,6 +949,7 @@
 static void
 METAL_SetTextureScaleMode(SDL_Renderer * renderer, SDL_Texture * texture, SDL_ScaleMode scaleMode)
 { @autoreleasepool {
+    METAL_RenderData *data = (__bridge METAL_RenderData *) renderer->driverdata;
     METAL_TextureData *texturedata = (__bridge METAL_TextureData *)texture->driverdata;
 
     if (scaleMode == SDL_ScaleModeNearest) {