Internal cleanup in the GL renderer

Separate the image cache from the texture cache.
The latter needs additional data in the key. The two have less in common that it seemed.

This also fixes a small issue with `CachedImage::upload_to_gpu` that for
SVGs would return a decoded (rendered) SVG image, instead of a texture.
That was harmless because we always call `ensure_uploaded_to_gpu`
later, but now we assert that the texture cache only holds textures,
because clear() (former remove_textures()) just clears everything.

sixtyfps_runtime/rendering_backends/gl/lib.rs

@@ -902,7 +902,7 @@ impl GLItemRenderer {
                     let image = source_property.get();
                     let image_inner = (&image).into();
 
-                    ImageCacheKey::new(image_inner, Some(image_rendering))
+                    TextureCacheKey::new(image_inner, image_rendering)
                         .and_then(|cache_key| {
                             self.graphics_window
                                 .texture_cache