Add and save "flipY" option to export dialog

[GoogleCodeExporter]

What steps will reproduce the problem?
1. Load a texture or normal map from non DDS format
2. Save as DDS file
3. In some content pipelines (e.g. threejs/webgl/game mods) that by default 
flip the image data the texture appears flipped now

What is the expected output? What do you see instead?
Some content pipelines expect that textures will be flipped in Y direction 
while loading or in webGL while uploading. Since DXT can't be flipped (?), you 
need to manually flip textures before saving, or change your content pipeline 
to always expect the texture origin at the top left (e.g. flip texture 
coordinates).
A convenience function in the export dialog like a "flipY" checkbox would be 
great and save time :)

What version of the product are you using? On what operating system?
3.0.1 windows 64 bit

Please provide any additional information below.

Original issue reported on code.google.com by [email protected] on 10 Feb 2014 at 1:00