Per various offline discussions (and some online, eg #5474), our OpenGL backend has many limitations/problems and is arguably of dubious value even if implementation issues are fixed. We should really decide whether we want to improve it or officially deprecate/remove them.

Strawman proposal to start a discussion:

• Designate these backends as 'deprecated' as of Halide 11
• Remove them entirely in Halide 12
• Take the work-in-progress Vulkan backend and devote resources to it with the intent of getting it useful for Halide 12