more work on correct rectangle handling

We now render black borders around rectangles and correctly handle
rectangles with just fill or just borders.

Still to be done:

* correct border style handling
* correct border color handling (maybe switch GLSL program)
* gradient handling
* still using GL_QUADS
* border line widths
* transparence

Change-Id: I3912ccf3dc5df686142a5d1758ee9b97fe435d7b