The case of calling get_current_point() on empty paths has not been
handled and caused panics.

SolidSource expects a premultiplied color.

…n ops

push_layer_with_blend() comes at a performance cost, so we only use it
on blend modes that require it.

From `addColorStop(offset, color)`s spec:
If multiple stops are added at the same offset on a gradient, they must
be placed in the order added, with the first one closest to the start of
the gradient, and each subsequent one infinitesimally further along
towards the end point (in effect causing all but the first and last stop
added at each point to be ignored)

https://www.w3.org/html/test/results/2dcontext/annotated-spec/canvas.html#testrefs.2d.gradient.interpolate.overlapu

Some information of `canvas_data::Pattern` was lost by
converting it to `raqote::Source` due to the fact that Raqote
did not store this information (e.g. linear gradient's start/end
points). We introduce another layer to keep this information for later
use (like in `is_zero_size_gradient()`).