Plea for a stock_img() alpha

[CommonClimate]

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.

[rcomer]

stock_img passes the image to imshow, and imshow handles alpha. So it looks to me that it might indeed just be a case of exposing the keyword, though I am certainly no expert here.

cartopy/lib/cartopy/mpl/geoaxes.py

Lines 1012 to 1014 in 7f132a5

	return self.imshow(imread(fname), origin='upper',
	transform=source_proj,
	extent=[-180, 180, -90, 90])

[dopplershift]

@rcomer That looks correct to me.