Clean up a bunch of v_video.c functions that previously exhibited unfortunate...

Clean up a bunch of v_video.c functions that previously exhibited unfortunate side effects when run in non-green resolutions.

* V_DrawFixedPatch and ilk:
	* Change the offset of V_FLIP so it's not one screen-pixel off where its non-flipped sprite would have started being drawn from.
	* Write to x and y as well as desttop so that anti-screen-overflow checks later in the function behave properly with non-green resolutions.

* V_DrawFill:
	* Reduce number of operations performed upon `c`.

* V_DrawString and ilk:
	* Offset the left and right boundary checks in non-green resolutions such that you can actually draw stuff to the left of basevid screen x coordinate 0.