Plugin build targets do not depend on GLEW include dirs when required

The OCIO color pipeline plugin and the media reader plugins all indirectly depend on the GLEW include directories (via `include/xstudio/ui/opengl/shader_program_base.hpp`). However, none of the plugin targets account for this, so they all fail to build if GLEW is not installed in the standard system locations.

Inspect the plugin build-target definitions and include/xstudio/ui/opengl/shader_program_base.hpp first; trace how the GLEW include directories are propagated. Verify that the OCIO color pipeline and media reader plugin targets build when GLEW is outside standard system locations.

Written by the indexing model from the issue text.