This reverts commit d6092fae27.
This change actually makes transitions start, and our code for stopping
transitions is just bogus, so we just keep re-starting them over and over, which
is not good.
Use background-size, background-position properties to render
CSS gradients.
Some cleanup in display_list_builder.rs related to gradient
calculations.
Adds two wpt tests for tiled gradients.
Note: For now even gradients with background-repeat: no-repeat
are repeated. Sometimes the gradient is not repeated everywhere.
Enable vars-background-shorthand-001.html CSS test.
