(frontend) Add support for frontend specific video drivers
(Menu) Implement stub functions for shader manager functions in menu_common_backend.c
Merge pull request #1022 from heuripedes/frontend-drivers

(frontend) Add support for frontend specific video drivers
(GLUI) Silence warnings
(frontend.c) Fix include
(Lakka) Display a black background if no png background supplied
(Nearest Resampler) C89 build fixes
Merge pull request #1024 from lakkatv/lakka

(Lakka) Display a black background if no png background supplied
Merge pull request #1023 from heuripedes/frontend

(frontend.c) Fix include
Split up driver_menu.h into three separate header files -
for menu display API, menu backend API, and menu driver general API
Move audio_frame_float typedef struct to resampler.h
Silence warning 'variable is used before its value is set'
Don't rely on retro_time_t typedef for menu_driver API
(Lakka) Fixes
(d3d_ctx.cpp) Add HAVE_EGL check
(string_list.c) Include posix_string.h