Fix alpha on some GDAL sources

[manthey]

This affected sources with a projection that have an explicit alpha channel.

Fixes girder/django-large-image#39

[banesullivan]

Turns out this didn't completely fix the issue:

In the following example, the alpha is still improperly handled

import large_image

path = '/Users/bane/Desktop/rasters/Django-Large-Image/4-ortho-rgb-cog.tif'

src = large_image.open(path, projection='EPSG:3857', style={
 'bands': [
    {'band': 1, 'palette': 'R'},
    {'band': 2, 'palette': 'G'},
    {'band': 3, 'palette': 'B'}
  ]
})
thumb = src.getThumbnail(encoding='PNG')[0]
thumb