Handle GFX CapsAdvertise re-advertisement from Windows clients

Nicolas Blackburnrequested to merge
stormlord/krdp:fix/caps-readvertise into master

Windows mstsc sends CapsAdvertise twice during connection setup: once during initial GFX channel establishment and again after confirming. If the second CapsAdvertise arrives after capsConfirmed is already true, the surface state becomes inconsistent because the GFX channel has been reset but the server still references the old surfaces.

This causes rendering artifacts or a black screen.

Fix by detecting re-advertisement (capsConfirmed is already true), resetting the surface state and pending frames, and re-confirming caps. This allows the GFX channel to be cleanly re-established.

Tested with: Windows 11 mstsc → KDE Plasma 6.6.1 (Wayland)

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