emulator: opengl: Make render_api.h C-compatible

This change modifies the function declarations under
developement/tools/emulator/opengl/host/include/render_api.h
to make them callable from C.

This is preparation work for integrating the rendering library
into the emulator. The plan is to dlopen() the library dynamically
and using non-mangled function names makes using dlsym() both
easier and more portable.

Change-Id: I34656ea4618dbb989fb6ff78df43e9bfb38a7799
2 files changed