This changelist simplifies default color handling in GLSL, making the renderer responsible for generating fallback textures when images are not found on disk, and removing associated dynamic branches in GLSL shader code.

Two advantages of this simplification are:
- Hydra Storm and other MaterialXGenGlsl integrations can now render materials that reference 1x1 images without needing special-case logic.
- The render performance of MaterialXGenGlsl shaders that reference images is slightly improved, as dynamic branches exact a performance cost in some situations.

This new logic doesn't yet handle color space differences between missing images and default colors, and we should address this across languages in a future improvement.