Add a test for #2945.

Add more alpha gradient test cases

* Add a vertical linear gradient. This bypasses AGG and uses a faster
code path, which works.
* Add gradients drawn in a ClipToPicture() context. This uses an
* unpacked scanline rasterizer, which works.

This hints to the problem being the use of a packed scanline rasterizer,
with agg gradients touching the alpha channel.

More test cases for gradients with alpha channel

* Swapping the opaque and transparent ends leads to different results,
* Linear gradients are also affected.

Add testcase from #2946.

Add a test for radial gradients.

* This shows that the radius is ignored and all gradients are drwan with
a radius of 100.

Add a test for #2945.

Add more alpha gradient test cases

* Add a vertical linear gradient. This bypasses AGG and uses a faster
code path, which works.
* Add gradients drawn in a ClipToPicture() context. This uses an
* unpacked scanline rasterizer, which works.

This hints to the problem being the use of a packed scanline rasterizer,
with agg gradients touching the alpha channel.

More test cases for gradients with alpha channel

* Swapping the opaque and transparent ends leads to different results,
* Linear gradients are also affected.

Add testcase from #2946.

Add a test for radial gradients.

* This shows that the radius is ignored and all gradients are drwan with
a radius of 100.