We want to use the same line decoration (dashed, dotted, wavy) shader code for
both horizontal and vertical lines, so it makes sense for them to use a
coordinate system that has been rotated (transposed, actually) so that .x always
runs parallel to the line being decorated, and .y is always perpendicular.

Before this patch, we passed the orientation enum as a vertex attribute, used a
switch to swap coordinates in the vertex shader, and then swapped them again in
the fragment shader.

This patch trades the orientation for a f32 'axis select' vertex attribute, and
uses mix to swap them in the vertex shader. Then no consideration is necessary
in the fragment shader: the vLocalPos varying is already in the appropriate form.

Since get_line_decoration_sizes is already thinking in terms of line-parallel
coordinates, it might seem like a good idea for decoration jobs to simply use
line-parallel coordinates throughout. However, this actually results in more
swapping and opportunities for confusion: much of the CPU work is concerned with
the rectangle the decoration's mask occupies in the texture cache, which is
axis-aligned.