[PD] flip image in glsl

[Patrice Colet]

Hello, 

 how is it possible to flip upside down an image in glsl example 05.multitexture

I tried to implement this in fragment program:

http://stackoverflow.com/questions/9857089/flip-upside-down-vertex-shader-gles

but it doesn't work. The only thing I can do is changing texture scale and position, but I couldn't do it with negative values
like it would be done in pix_coordinates.

In fact when I use pix_multiimage before pix_texture, the image gets flipped, I don't know why...

Colet Patrice