rawrgb images support

[Evgeny Zajcev]

Hello!

At the moment there is no possibilities to load image as raw RGB data in GNU Emacs, even if using nice 'imagemagick image type.

The problem is that with raw rgb format the filename hint is not enough for imagemagick to correctly interpret blob data.  size and depth also must be provided.

I created tiny patch, that adds this functionality

https://github.com/zevlg/emacs/commit/17d85ca9e1c7c071602ef270c9480cdb542fb53d

Thanks

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lg