MS-DOS: Fix custom colors in VESA BIOS mode

The basic colors are being displayed as black. Colors specified as RGB
are not being converted to the pixel format for the current mode.  This
manifests as walls in dungeon branches being drawn in black when the
symbol set is IBMgraphics, and in the wrong color when Enhanced1 is in
use and the display mode uses 15 or 16 bits per pixel. A particular
mode that shows this bug is 1024 by 768 under DOSBox.

sys/msdos/vidvesa.c

@@ -691,9 +691,16 @@ vesa_xputg(const glyph_info *glyphinfo, const glyph_info *bkglyphinfo UNUSED)
     }
 #endif
     if (vesa_pixel_size > 8 && glyphinfo->gm.customcolor != 0) {
-        /* FIXME: won't display black (0,0,0) correctly, but the background
-           is usually black anyway */
-        attr = glyphinfo->gm.customcolor | 0x80000000;
+        attr = glyphinfo->gm.customcolor;
+        if (attr & NH_BASIC_COLOR) {
+            attr &= 0x0F;
+        } else {
+            struct Pixel p;
+            p.r = (unsigned char)(attr >> 16);
+            p.g = (unsigned char)(attr >>  8);
+            p.b = (unsigned char)(attr >>  0);
+            attr = vesa_MakeColor(p) | 0x80000000;
+        }
     }
 
     row = currow;