SDL: New OSD and Shader Support

[ChrisDeadman]

I want to use 86Box on a Raspberry Pi without any X or Wayland, just boot into minimal init and 86box with SDL.
Since I was used to CRT shader on QT build, I was not happy to find out shaders are not supported in SDL mode, so I have added support for SDL shaders in this branch.

However, this pulls in gles2 dependency that wasn't required before, so I decided to start this discussion.
Also, this applies the shader to the OSD (doesn't break immersion with e.g. CRT shader), but changes behavior since that was not the case before, new way is just my personal preference.

Also, while adding shader support and using SDL backend, I found the OSD to be quite lacking. It is better than nothing, but imo this new one I've created based on imgui looks and functions better:

I am working on it in this branch, but should be discussed before any PR.

[OBattler]

I don't really see any problems with what you're doing.

[ChrisDeadman]

For everyone interested: It's merged now. Thanks @OBattler for accepting.
Unfortunately, it is not working on Mac at the moment, but I will look into it when I get the time.
For Linux SDL build it can be built setting both -DUSE_SDL_SHADER=ON and -DUSE_IMGUI=ON; OSD is opened with RCTRL+F11.
Let me know if there's something that could be improved.