[WIN32K:NTGDI] Mark 2-color indexed palettes as monochrome

This is mostly an optimization, as translation from RGB to mono are much faster than generic indexed palettes.
It exposes a broken RGB-to-mono translation though, which previously was hidden for DIB sections, which didn't mark the palette as mono, therefore taking the slow index path, which worked correctly.

win32ss/gdi/ntgdi/palette.c

@@ -149,6 +149,9 @@ PALETTE_AllocPalette(
         /* Check color count */
         if ((cColors == 0) || (cColors > 1024)) return NULL;
 
+        /* Mark 2 color indexed palettes as monochrome */
+        if (cColors == 2) iMode |= PAL_MONOCHROME;
+
         /* Allocate enough space for the palete entries */
         cjSize += cColors * sizeof(PALETTEENTRY);
     }