webP and webM support in LiveCode

[hh]

> Al wrote:
> Yes, LiveCode shell control is a real godsend for
> working with command line programs, but we could
> not use webP compressed images within our stacks
> unless LC engine could decode them back to RGBA
> images within LiveCode.

So what's the problem?

Decode them with shell to a (temporary) file

On Mac:
put shell(/usr/local/bin/dwebp input.webp -o output.png)
On Linux:
put shell(/usr/bin/dwebp input.webp -o output.png)

Then set the fileName to the decoded file (or import the
temporary decoded file).