LibWeb/CSS: Improved implementation of background-blend-mode

This is a improved version of a73cd88f0c
The old commit was reverted in 552dd18696

The new version only paints an element into a new layer if background
blend modes other than normal are used. The rasterization performance
of most websites should therefore not suffer.

Co-Authored-By: Alexander Kalenik <kalenik.aliaksandr@gmail.com>

Libraries/LibWeb/Painting/StackingContext.h

@@ -12,26 +12,6 @@
 
 namespace Web::Painting {
 
-#define ENUMERATE_MIX_BLEND_MODES(E) \
-    E(Normal)                        \
-    E(Multiply)                      \
-    E(Screen)                        \
-    E(Overlay)                       \
-    E(Darken)                        \
-    E(Lighten)                       \
-    E(ColorDodge)                    \
-    E(ColorBurn)                     \
-    E(HardLight)                     \
-    E(SoftLight)                     \
-    E(Difference)                    \
-    E(Exclusion)                     \
-    E(Hue)                           \
-    E(Saturation)                    \
-    E(Color)                         \
-    E(Luminosity)                    \
-    E(PlusDarker)                    \
-    E(PlusLighter)
-
 class StackingContext {
     friend class ViewportPaintable;