Add transparency option for raster tiles

[dazza-codes]

Is it possible to add a transparency override for raster tiles? When the mbtiles contain RGBA tiles in PNG format, where the alpha channel specifies a zero opacity (fully transparent), it would be useful to apply a CLI option to override that opacity for the viewer.

It might be possible to create a quick PR for this, with a pointer to the relevant code that could be modified to support this feature.