Speed up drawing of the rounds for polylines.

The algorithm is the same as is used by al_calculate_arc, but that function itself could not be used as it was poorly designed and doesn't mesh well with the emitting code used for polylines.
Be a bit paranoid and draw the last triangle with the precise end angle.

This doesn't actually happen with the new code, but the old code produced gaps because of this issue.
Update the "test filled polygon [sw]" unittest to pass with the new round drawing code.

The old code was actually wrong as it produced gaps between the miter and the line, so this is an overall improvement.
Nick Trout added init/shutdown stubs to iOS native dialog addon
so it would compile.