Updates bitmap sizing and logic checks for adding/removing bitmap

[kmatch98]

After merging @FoamyGuy latest changes, here are a couple of more edits to correct the bitmap sizing.

Also, I updated _update_text to call _update_background_color to reuse code.

I updated the boolean checks for the bitmap inserting/appending/deleting bitmaps at the end of _update_background_color to respond to zero-width and zero-height bitmap sizing.

[kmatch98]

@FoamyGuy I think I merged your changes and my udpates into here. Please review the changes and test.

I made these updates to resolve a couple of issues I observed in my testing:

• Upon creating a text box, the bitmap would not get created on the initial loop.
• When changing the text to '' there was still a bitmap showing.
• There was an error calculating the bitmap width in the boolean checks. You had used len(text) this is # of characters, rather than self._boundingbox[2] this is the number of pixels for width.
• I added another boolean check for the bitmap height > 0. With negative padding, this could be likely.
• I made _update_text just call _update_background_color to prevent repeating code

[kmatch98]

I'm going to merge this PR and then we can make any further updates with another PR.