glamor: Fall back for mixed depth 24/32 in glamor_set_alu

For ALUs which may leave the alpha channel at values other than 1.0.

Closes: https://gitlab.freedesktop.org/xorg/xserver/-/issues/1615

v2:
* List safe ALUs instead of unsafe ones

glamor/glamor_pixmap.c

@@ -136,6 +136,21 @@ glamor_set_alu(DrawablePtr drawable, unsigned char alu)
         glDisable(GL_COLOR_LOGIC_OP);
         return TRUE;
     }
+
+    switch (alu) {
+    case GXnoop:
+    case GXor:
+    case GXset:
+        /* These leave the alpha channel at 1.0 */
+        break;
+    default:
+        if (glamor_drawable_effective_depth(drawable) == 24 &&
+            glamor_get_drawable_pixmap(drawable)->drawable.depth == 32) {
+            glamor_fallback("ALU %x not supported with mixed depth\n", alu);
+            return FALSE;
+        }
+    }
+
     glEnable(GL_COLOR_LOGIC_OP);
     switch (alu) {
     case GXclear: