GNOME Mutter 51.rc Introduces Cross-GPU Buffer Scan-Out and Last-Minute Tweaks

Release candidates for GNOME Shell and Mutter 51.rc add cross-GPU buffer scan-out to improve multi-GPU setups (e.g., hybrid laptops), plus features like stylus eraser button configuration, Night Light on GPUs without GAMMA_LUT, and a configurable background policy. Also includes fixes for SVG cursor hardware cursor usage, primary monitor reporting across multiple displays, HDR SDR desaturation, external monitor hot-plug hangs, and direct scan-out without DMA-BUF; Shell 51.rc adopts the Glycin library for background images and rounds out with additional fixes.