Update README and build files for OpenGLCompute requirements

[steven-johnson]

Support for OpenGL was removed in #5626 for Halide 12, but support for OpenGLCompute is being kept; the requirements for OpenGLCompute (in terms of runtime dependencies, etc) are similar to OpenGL but it's not clear if it's identical or not, so various README and Make/CMake deps may need updating. At the time of this issue being opened, that's not clear, because OpenGLCompute isn't being tested (#5627) due to LLVM symbol exporting (#4651); thus this is a placeholder to ensure that we circle back to address this before Halide 12 is released.