xserver: fix RGB mask handling

On FreeBSD 13.0-CURRENT for PowerPC64 big-endian (BE), X was
crashing in some cases. For instance, when twm was started
and the background was clicked to open its menu, X crashed
with a segmentation fault, trying to dereference a null pointer
at CreatePicture().

There were 2 issues with xorg-server handling of RGB masks that
caused the pointer above to be null and thus the crash:
- wrong use of ffs() to get the RGB offsets from the masks
- overflow when shifting a 16-bit integer

This change fixes both issues. They happen when the system is BE
but has a video adapter using a little-endian (LE) ARGB32
framebuffer. In order to display the correct colors, this setup
requires a BE RGBA32 color format to be used by X, by setting
the RGB masks appropriately, that didn't work properly because of
the issues above.

hw/xfree86/common/xf86Helper.c

@@ -729,9 +729,9 @@ xf86SetWeight(ScrnInfoPtr scrp, rgb weight, rgb mask)
         scrp->mask.red = mask.red;
         scrp->mask.green = mask.green;
         scrp->mask.blue = mask.blue;
-        scrp->offset.red = ffs(mask.red);
-        scrp->offset.green = ffs(mask.green);
-        scrp->offset.blue = ffs(mask.blue);
+        scrp->offset.red = ffs(mask.red) - 1;
+        scrp->offset.green = ffs(mask.green) - 1;
+        scrp->offset.blue = ffs(mask.blue) - 1;
     }
     return TRUE;
 }

render/picture.c

@@ -523,12 +523,12 @@ PictureMatchVisual(ScreenPtr pScreen, int depth, VisualPtr pVisual)
                     return format;
             }
             else {
-                if (format->direct.redMask << format->direct.red ==
-                    pVisual->redMask &&
-                    format->direct.greenMask << format->direct.green ==
-                    pVisual->greenMask &&
-                    format->direct.blueMask << format->direct.blue ==
-                    pVisual->blueMask) {
+                if ((unsigned long)format->direct.redMask <<
+                        format->direct.red == pVisual->redMask &&
+                    (unsigned long)format->direct.greenMask <<
+                        format->direct.green == pVisual->greenMask &&
+                    (unsigned long)format->direct.blueMask <<
+                        format->direct.blue == pVisual->blueMask) {
                     return format;
                 }
             }