OpenGL-OpenCL interop, which as far as I understand is governed by the cl_khr_gl_sharing [1, 2] extension, allows applications to use OpenGL buffer, texture and renderbuffer objects as OpenCL memory objects.
I use it in a video capturing utility to perform OpenCL-accelerated color conversions of OpenGL textures. Looks like it's not supported by Mesa's OpenCL implementation.
I'm not at all familiar with the Mesa codebase so I'm not sure how much effort supporting this would require.
Just wanted to note that we are also missing EGL_KHR_cl_event2, AFAIK this is the only missing bit from promoting our EGL to 1.5.