Description

I am delighted that .add_feature() now allows for transparency via an alpha parameter; this goes a long way to decluttering busy maps. Unfortunately, the same feature is not available for stock_img(), and I was wondering why. I am admittedly ignorant of the way the background is implemented in Cartopy, but given that a lot of matplotlib objects have an alpha parameter, I was hoping this would be a matter of exposing it at the API level. Is this thinking too simplistic?

Proposal

ax.stock_img(alpha=0.5) reduces background opacity by 50%, for instance.