in the fragment shader.

This reintroduces the clip-in rect in the FS. Since we'll need it anyway
when we have arbitrary matrix/clip stacks, this seems harmless.

Color strips for non-axis-aligned linear gradients are submitted as
axis-aligned rects centered at the appropriate point, just as
axis-aligned ones are. However, we reuse one of the vertex fields to
store the rotation angle. The vertex shader detects this condition,
rotates the rectangle around its midpoint, and passes the clip rect on
to the fragment shader.

Additionally, this patch eliminates the ugly (-10000,10000) spans for
linear gradient strips that we had before. It was necessary to tighten
this up in order to avoid needless fragment shader invocations in the
non-axis-aligned case, so I went ahead and fixed it everywhere.

Closes #161.