Support mid scanline LCD register writes.

[Senryoku]

Pocket Demo, and probably some games, have some graphical glitches due to mid scanline writes to the LCD registers.
For example, Pocket Demo seems to change the selected tile set (LCDC BGWindowsTileDataSelect) mid scanline to display its spiralling title.

We could call render_line for each write, before actually modifying the register, to render the previous pixels and mark them as done (just store the index of the last drawn pixel). It already requires some refactoring, but totally doable.
However, to be totally accurate, we would need to correctly split the process between background/window and sprites rendering, and figure out the precise timing of all these operations...

This is probably not going to happen.