crash from bad inputs to VGA library functions

[Jaxartes]

A crash sometimes occurs in the "ZPUino_VGA_Adapter/Demo" example code, because it can (depending on screen dimensions) pass out-of-bounds values to gfx.fillRect() and gfx.fillRoundRect() and this results in writing to memory outside the screen area.

It's subject to debate whether this should be fixed in library functions should fix this by checking their inputs (which may have a performance impact) or the "ZPUino_VGA_Adapter/Demo" example by avoiding the bogus values. Or both.

It doesn't always happen, it's one of those "Heisenbugs" which appear/disappear randomly as a result of any changes made in the code. I saw this bug on a Papilio Pro with 800x600 graphics after applying my own workaround to issue #24.