demos/pixeltest: Remove Line struct.

Not really used/necessary.

Reviewed-by: Brian Paul <brianp@vmware.com>
demos/pixeltest: Adapt the example for points too.

This example is very useful to understand the rasterization of lines.
And with this change, points too.

Adding a key / command-line option to switch modes is left for a future
opportunity though.

Reviewed-by: Brian Paul <brianp@vmware.com>