Rendering of empty special tiles sometimes results in visual artifacts

[chrahunt]

As mentioned in #191 and here, when using this tile texture, there are small bars visible beneath empty bomb tiles and empty flag tiles. Example:

The top of the tile in the row below these tiles in the sprite images are getting picked up somehow. I checked the y offset of the files below empty bomb and empty flag in that texture pack and they are correct (120).

[chrahunt]

It looks like the artifact only appears when the fractional part of y > 0.5, so it may be some anti-aliasing effect that merges adjacent pixels from the source image with the existing destination pixels. Having an intermediate representation that only includes the images (like drawing the images in isolation to their own offscreen canvases) and sourcing from that should fix it. It may also be better to transition to something like Pixi, which will clean up a lot of the rendering code and allow re-use of parts of the TagPro renderer.