Add 24-bit color support to exaGetPixmapFirstPixel

It appears that people who developed EXA forgot that there used to be
graphics devices that used 24-bits (3 bytes) instead of 32-bits (4 bytes)
in order to display one pixel. The lack of 24-bit color support inside
exaGetPixmapFirstPixel causes SiS 6326 to crash when running Xfce since
SiS 6326 does not support 32-bit color.

Signed-off-by: Kevin Brace <kevinbrace@gmx.com>

exa/exa_unaccel.c

@@ -703,6 +703,7 @@ exaGetPixmapFirstPixel(PixmapPtr pPixmap)
 {
     switch (pPixmap->drawable.bitsPerPixel) {
     case 32:
+    case 24:
     {
         CARD32 pixel;