Refresh Fix for TileGrid

[FoamyGuy]

@ladyada
Resolves: #150

I believe the root cause of the issue is the refresh thread running on it's own cycle in the background while the user code is manipulating Groups, TileGrids, Bitmaps etc. which cause refresh to be needed. I think there is a period of time during the refresh process after _get_refresh_areas() has been called for a given object, and before _finish_refresh() gets called for that same object. During that window of time if the user code manipulates the object making it need to be re-drawn it won't actually get drawn because _finish_refresh() gets called first which makes the next call to _get_refresh_areas() not return any areas to draw because _finish_refresh() clears / resets the dirty area to empty.

This change resolves it by adding a _needs_refresh bool that gets set to True whenever any change that triggers a refresh occurs. Then inside _finish_refresh() we only do the finish logic when that bool is False. The bool is flopped to False in _get_refresh_areas() once we know that our triggered refresh is in process.

I tested this fix successfully with the reproducer from the issue.

Unrelated to the issue above, I also removed a duplicated _finish_refresh() function inside of Bitmap. The remaining function is here: https://github.com/adafruit/Adafruit_Blinka_Displayio/blob/main/displayio/_bitmap.py#L294

[makermelissa]

Looks good. Makes sense the way you did it because we are using threads and the code I ported from circuitpython doesn't have that challenge.