glteximage is used in gl common sources and it isn't compatible with gles. These functions are used in a few tests and prevent the harness from running gles.
The alternative to this is that we don't delete texture data after upload and always keep a system memory copy around. We discussed this problem quite a bit for Direct3D9 because like original GM things like the screen saver could destroy surfaces and textures and necessitate a device reset. However, keeping a system memory copy is also trickier to keep synchronized with what's actually on the GPU plus it decreases the total RAM available for the game to use.