GlfBaseTexture, automatic mipmapping and core OpenGL profile

[ShaderManager]

Description of Issue

GlfBaseTexture uses old OpenGL1.4 functionality to generate mipmap chain, which is deprecated and removed in core OpenGL 4.5 profile. For example, this leads to broken texture object for default domelight and following inability to calculate IBL tables properly.
https://github.com/PixarAnimationStudios/USD/blob/master/pxr/imaging/glf/baseTexture.cpp#L368
Possible fix is to use glGenerateMipmap instead.

Steps to Reproduce

1. Create OpenGL 4.5 context with core profile bit
2. Try to use GlfBaseTexture with automatic mipmap generation.

Package Versions

Tested with 20.05.

[jilliene]

Filed as internal issue #USD-6009