add line-fill plot calls

[stevesims]

implements the line fill plot codes in blocks 0x48, 0x58, 0x68 and 0x78

NB this is a preliminary/experimental version, has not yet been fully tested, and is a slow/inefficient implementation

[stevesims]

so performance for these plot calls is poor, however they do appear to work. later optimisation can be done by moving this functionality inside vdp-gl taking a slightly different approach.

as these plots work, they'll get added. optimisation can come later.