Parts of the EmuGL layer are generated with a tool called emugen (see external/android-emugl/host/tools/emugen).
To generate the source again after a modification simply call
$ scripts/update-emugl-sources.sh --emugen=<path to emugen>/emugen
The definition of the various attributes/types/functions can be found in
- external/android-emugl/host/libs/renderControl_dec
- external/android-emugl/host/libs/GLESv1_dec
- external/android-emugl/host/libs/GLESv2_dec