Clip raster tiles to avoid showing more than one tile in the same location

[kkaefer]

After #8769 got merged, our handling of sparse raster tilesets changed. Sparse raster tilesets don't have an image for every location within their bounds, e.g. because the image would be empty.

We've never had real support for sparse tilesets, but I believe we could fix this by cutting holes into the parent tiles in the locations where we have child tiles loaded. Currently, we use one quad to draw the entire tile, but we could dynamically create buffers that consist of only the quads required to cover the part of the screen that isn't covered by other map tiles yet.

This would fix #7760 and #8678.