Platform: fix WindowlessGlxApplication context creation and destruction.

The variable wasn't initialized so it exited with assertion failure,
moreover the context wasn't deleted on destruction, causing memory leak.
DebugTools: saner default color.

Default framebuffer clear color is black, thus having black debug
renderers by default isn't much useful.
Math: cleaned up plane/line intersection.

The function now accepts starting point and direction of the line (not
starting and ending point). Also improved the documentation and test a
bit.