primitives: Delete al_draw_polygon_with_holes.

Despite the name, all al_draw_polygon_with_holes does is draw multiple
polygons.
primitives: Delete stride-less version of al_draw_polyline*.

This saves us the embarrassment of an *_ex function.
primitives: Simplify interface of al_triangulate_polygon and al_draw_filled_polygon_with_holes.

Also improved their documentation.

Give some extra feedback to the "add holes" mode of ex_polygon.c.
ex_polygon: Don't reset the mode when R is pressed.
ex_polygon: Use enum for add_hole mode.
ex_polygon: Use builtin font.
tests: Add test for al_draw_filled_polygon_with_holes.