Fix three bugs in the equals() implementations for css gradients.
1. Linear gradients were considered equal if the first gradient has no x and y
position and the second has no x but does have y.
2. Same as 1, for radial gradients. (This doesn't happen in practice as
CSSParser::parseRadialGradient rejects such input, so no test for this case.)
3. Radial gradients without x and y position weren't considered equal even if
Found by clang's -Wunused-value in combination with