point3d: Fix header alignment
quad: Add rect_t initializer

It would be more convenient to be able to initialize a quad from a rect
in a consistent way.
matrix: Use the right order of points when transforming

Follow the corners in clockwise order.
matrix: Missing 'z' scaling factor

Typo: we're reusing the 'x' scaling factor for the 'z' row.

Bug reported by: Mattias Bengtsson <mattias.jc.bengtsson@gmail.com>