render: add OpenGL resource caching; optimize texture handling

Mildly improves rendering performance by caching OpenGL resources.
Namely:

- Cache shader program, VBO/EBO, and textures per output
- Automatically free image data after GPU upload
- Force RGBA format for consistent texture handling
- Track texture state across output changes
- Add texture invalidation on image changes

This reduces the memory usage by a solid 30MB, but it's still not quite
enough. I expect (or rather, hope) that we can cut it by half.

Signed-off-by: NotAShelf <raf@notashelf.dev>
Change-Id: I6a6a6964eebc783c5bc07b1fef7548a8d49f529c

src/core.c

@@ -87,6 +87,14 @@ static int assign_wallpaper_to_output(chroma_state_t *state,
     return CHROMA_ERROR_IMAGE;
   }
 
+  // Check if image changed and invalidate texture cache if neceessary
+  bool image_changed = (output->image != image);
+  if (image_changed && output->image) {
+    chroma_output_invalidate_texture(output);
+    chroma_log("DEBUG", "Image changed for output %u, invalidated texture",
+               output->id);
+  }
+
   // Assign image to output
   output->image = image;