Fix merge_images for strided images

[scottstanie]

Two changes need to fix this:

1. we were passing strides to get_combined_bounds_nodata, even thought we already used it in get_downsampled_vrts. this meant it was double counting the strided output resolution, leading to that weird rounding error in @taliboliver 's example
2. Now we do a gdal_translate after merging, since gdal_merge has no resampling algorithm to guarantee the output bounds.