move prev,line swap to after putting data into bitmap

[FoamyGuy]

@ladyada

Resolves: #91

This swap of prev and line was prior to data from line being inserted into the Bitmap. That resulted in the first row being blank because prev was empty on the first iteration. And led to the last row being "pushed off the bottom" and not rendered.

Fix tested on Feather S3 TFT 9.2.1 with this slightly modified reproducer from the issue:

import displayio
import adafruit_imageload

import board
display = board.DISPLAY

display.root_group = displayio.Group(scale=4)

image, palette = adafruit_imageload.load(f"test_pattern.png")
display.root_group.append(displayio.TileGrid(image, pixel_shader=palette, x=2, y=2))

bmp_image, bmp_palette = adafruit_imageload.load(f"test_pattern.bmp")
display.root_group.append(displayio.TileGrid(bmp_image, pixel_shader=bmp_palette, x=20, y=2))

while True:
    pass

[jepler]

This is kinda subtle. would you be able to add a test that probes for correct pixel values? We have some tests already, at e.g., https://github.com/adafruit/Adafruit_CircuitPython_ImageLoad/blob/main/tests/test_pbm_load.py that you might be able to model it after. (though I didn't see any of them actually checking pixel values)

[FoamyGuy]

@jepler yes, I've added that to my list. I'll create a separate PR for it and merge this so we can have the fix more immediately.

I hadn't looked into these tests much if at all before. I wonder if it'd be possible to use Blinka Displayio implementations of Bitmap and Palette instead of the mocked ones from tests.displayio_shared_bindings.py