Scrolling Pattern can have a size mismatch with fixture

[cibomahto]

Steps to reproduce:

1. Start patternpaint, select 'BlinkyTape', load example patterns
2. Select 'Tools->scene configuration'
3. Change the fixture to matrix-rows, and set the size to 8x8
4. Click ok to accept the changes, then 'no' to skip resizing the patterns
5. Note that the fixture size is drawn correctly on the editor and in the output preview, however the output preview (and any connected blinky) will only show data for the lefthand most column

Expected result:
The whole matrix will get updated data correctly

Note: This is because the scrolling matrix maintains a bogus 'size' feature that is used when retrieving the frame image (since it has to generate this from the source image).