preview-20170425

First preview release.

MacOS binaries provided by @underscorediscovery

Notable features:

• Loads common image formats (JPEG, BMP, PNG, TGA, BMP, GIF, etc.) using stb_image.
• Auto-generate mipmaps (box-filtered, nothing fancy).
• Fast ASTC, ETC1, BC1, BC3, and BC7 compression provided by ispc_texcomp. Or just stick with uncompressed RGBA.
• Output to KTX files.
• Convert multiple images to a 2D texture array, cubemap, or cubemap array.