testsuite: Adapt color-matrix testcase

Make it use an alpha value that is well defined, ie 0.4 instead of 0.5.

0.4 * 255 = 102
0.5 * 255 = 127.5

This avoids rounding issues where some math may cause the resulting
alpha value to be 127, and some other math ends up with 128.

testsuite/gsk/compare/color-matrix-identity.node

@@ -1,7 +1,7 @@
 color-matrix {
   child: color {
     bounds: 0 0 50 50;
-    color: rgba(255, 0, 0, 0.5);
+    color: rgba(255, 0, 0, 0.4);
   }
   matrix: none;
   offset: 0 0 0 0;