Introduce Texture class to dry up code

[mourner]

Abstracts our texture logic (which was repeated in many places) in a Texture class, reducing the amount of code and the error surface. A work in progress — just wanted to get this out to show the direction this is going and get some feedback.

Launch Checklist

• fix a few render test failures
• briefly describe the changes in this PR
• write tests for all new functionality
• document any changes to public APIs
• post benchmark scores
• manually test the debug page

[anandthakker]

@mourner nice! Approach looks good to me overall. One thing I wonder: would it be worthwhile to try to avoid having Texture keep the reference to the GL context, by having it passed in to setData, setImage, etc?

[jfirebaugh]

Added something similar in #5190; tracking future work in #5277.