Remove multiple flips of the V texcoord in MaterialXView (upon glTF load and when accessing textures)

[bhouston]

Fixes the issue identified here: #2881

The MaterialXViewer flips V texture coordinates of obis and glTF files upon load. It also flips the V texture coordinate when accessing textures. I believe both of these flips are incorrect. Neither should exist. This PR removes both of them.

It leads to simpler code but also now the UV space of a mesh does not have to be modified in order to render it.

Lastly, math operations in a flip UV space are not the same as in a non-flipped UV space. Thus this flipped UV space leads to incorrect math results:

prefliped V with math != post-flipped V with math

flipV(V) + 0.1 != flipV( V + 0.1 )
( 1 - V ) + 0.1 != 1 - ( V + 0.1 )
1.1 - V != 0.9 - V

And also pre flipped + post-flipped V doesn't equal no flipping:

flipV( flipV(V) + 0.1 ) !== V + 0.1
1 - ( 1 - V + 0.1 ) != V + 0.1
1 - 1 + V - 0.1 != V + 0.1
V - 0.1 != V + 0.1

[bhouston]

This was an incorrect PR. glTF's UV space is flipped compared to MaterialX/Blender.