discard any pixels with negative coordinates to avoid a panic in later code

[mutantbob]

I tried to draw some text at x,y = 1,1 , and got a panic inside batch.rs

It seems that the code to batch up pixels was converting an i16 to a u16, and when it came time to self.y_bottom + 1 fory=(-1 as u16) that cause an arithmetic overflow and a panic.

This patch discards any pixels with coordinates <0

[almindor]

Nice catch, thanks.

I've mostly abandoned this crate for mipidsi though. I'll apply this fix there too, but you might want to switch to using that since the st7789 will become deprecated soon.