glass panels: add the PlaneChecksum header decl (completes c9e25e5)

The dirty-skip commit staged the header via a lowercase path (l4vb16.h) while git
tracks it as L4VB16.h on this case-insensitive FS, so the PlaneChecksum declaration
was left out -- c9e25e5 as committed would not build from clean (L4VB16.cpp defines
SVGA16::PlaneChecksum, L4GLASSWIN.cpp calls it, but the header never declared it).
This adds the one-line decl.  HEAD now builds standalone.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

engine/MUNGA_L4/L4VB16.h

@@ -419,6 +419,12 @@ public:
 	// dwords; the image is written TOP-DOWN.  *outW/*outH receive the produced size.
 	void ExpandPlaneToBGRA(int mask, int paletteID, int monoTint, int rotateQuadrant,
 						   unsigned long *dst, int *outW, int *outH);
+
+	// GLASS dirty-skip (L4GLASSWIN, 2026-08-09): FNV-1a checksum of the shared
+	// gauge pixelBuffer, masked to the bits a given window can show.  Lets the
+	// glass repaint pump re-blit ONLY the windows whose plane actually changed
+	// (idle MFDs / static panels skip; the sweeping radar keeps updating).
+	unsigned long PlaneChecksum(int mask) const;
 };
 
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