Fix decode of 3-channel WEBP image data

[rgov]

The WebImageDecoder always returns a buffer of RGBA pixel values. If the caller is expecting RGB, a corrupted image is rendered. There was a TODO comment remarking on this:

geotiff.js/src/compression/webimage.js

Lines 35 to 36 in c0c5f76

	// TODO: check how many samples per pixel we have, and return RGB/RGBA accordingly
	// it seems like GDAL always encodes via RGBA which does not require a translation

This resolves the issue by removing the alpha channel if SamplesPerPixel is 3.

Fixes #373.

Before and after (disregard the overlay text):

[TheMrCam]

LGTM 👍 I actually encountered this for the first time recently—appreciate the fix!