cssvalue: Change color transitions to run premultiplied

Previously, the unpremultiplied values from the GdkRGBA were taken. Now
we premultiply the color values as specified by the CSS specs.
This is only relevant when transitioning with translucent colors.

An example is the halfway transition between transparent (0, 0, 0, 0)
and white (1, 1, 1, 1). Previously, all 4 values where transitioned
separately and the result was semi-transparent gray (0.5, 0.5, 0.5,
0.5).
By depending on the alpha value, the result is now semi-transparent
white (1, 1, 1, 0.5) which is what one would naively expect.

New reftest: color-transition

1 files changed, 18 insertions, 0 deletions

diff --git a/testsuite/reftests/color-transition.css b/testsuite/reftests/color-transition.css
new file mode 100644
index 0000000000..ac964b00b6
--- /dev/null
+++ b/testsuite/reftests/color-transition.css
@@ -0,0 +1,18 @@
+@import "reset-to-defaults.css";
+
+@keyframes foo {
+  100% { background-color: blue; }
+}
+
+GtkWindow {
+  background-color: #7F0080;
+}
+
+GtkBox {
+  background-color: red;
+}
+
+GtkBox GtkBox {
+  background-color: transparent;
+  animation: foo 100s steps(2);
+}