Follow naming convention for shader output attributes

The convention¹ is to name output attributes like <VarName><ShaderAcronym>Output. This commit renames variables which did not follow that rule.

The benefit of this rule is to allow optional shaders in between vertex and fragment shader.

1. https://gitlab.kitware.com/vtk/vtk/-/blob/master/Rendering/OpenGL2/glsl/readme.txt?ref_type=heads ↩