Guard against out of bounds pixel set

[FoamyGuy]

Resolves: #95

I've tested successfully with the image file in the issue, as well as all PNGs included in the examples directory in this repo all on a PyPortal Titano 9.2.4

[jepler]

I think this makes sense (say you have width = 31 (valid x coordinates 0..30 inclusive) and pixels_per_byte = 2. You will reach the point where x + pixel is 31, causing an IndexError on old line 116....

There don't seem to be any tests of png loading. perhaps there should be some.

[FoamyGuy]

I started writing a test for this, but it turns out that the Blinka DisplayIO implementation of Bitmap does not raise an IndexError if you attempt to set pixels outside of it's size so the test passes even in what would be a failure case on the microcontroller. Once a change is made in Blinka Displayio to align it with core we can have a test here will effectively check for this and ensure it stays working.