GL/GLES: split off GLES from GLTexture

[sarbes]

Description

This PR splits off the GLES part of GLTexture.

Motivation and context

No more ifdeffery (#25205).

How has this been tested?

GL and GLES compile and run fine.

What is the effect on users?

None.

Screenshots (if appropriate):

Types of change

• Bug fix (non-breaking change which fixes an issue)
• Clean up (non-breaking change which removes non-working, unmaintained functionality)
• Improvement (non-breaking change which improves existing functionality)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that will cause existing functionality to change)
• Cosmetic change (non-breaking change that doesn't touch code)
• Student submission (PR was done for educational purposes and will be treated as such)
• None of the above (please explain below)

Checklist:

• My code follows the Code Guidelines of this project
• My change requires a change to the documentation, either Doxygen or wiki
• I have updated the documentation accordingly
• I have read the Contributing document
• I have added tests to cover my change
• All new and existing tests passed

[sarbes]

Yeah, sorry. I've had a cleaner version I did not commit.

I've also improved the truncation behavior for GLES 3.0.