Support for OpenGL ES 2.0

[rmitton]

This PR extends the OpenGL backend to add support for OpenGL ES 2.0. The VAO feature the existing 3.0 backend uses is not strictly required, so can simply be #ifdef'd out in this case.

(the previous code required ES 3.0, which some desktop implementations do not support)

[rmitton]

I just noticed the following PR also covers this issue. I suspect this patch is simpler. Not sure whether to cancel my one or not.
#2351

(note that this code also requires IMGUI_IMPL_OPENGL_LOADER_CUSTOM to be set to <GLES2/gl2.h>)

[ddovod]

+1
I'm just trying to find a way to disable vao in imgui code so it can be used on macOS with opengl2 context with shaders, and vao is the only showstopper

[ocornut]

Thanks a lot Richard,

I just noticed the following PR also covers this issue. I suspect this patch is simpler. Not sure whether to cancel my one or not.

Unfortunately right now the other PR is in a barely usable state (lots of off-topic cruft and looks like an accidental attempt to hide the signal inside maximum amount of noise).

I am a surprised that just commenting out the VAO code makes it work, I must misunderstand GL but haven't dug into it, anyway it is great that we have a second and explicit confirmation about it!

I've got the right amount of information in this PR to proceed forward.

(the previous code required ES 3.0, which some desktop implementations do not support)
(note that this code also requires IMGUI_IMPL_OPENGL_LOADER_CUSTOM to be set to <GLES2/gl2.h>)

Could you clarify which implementation do or do not support ES 3.0?
GL back-ends have been a never-ending source of entangled troubles so I would like to gather and log every piece of explicit information I can.

Do you think we could rework the top part of the .cpp file to detect ES2 systems the same way we attempt to detect ES3? And therefore completely bypass the IMGUI_IMPL_OPENGL_LOADER_xxxx bits.

// iOS, Android and Emscripten can use GL ES 3
// Call ImGui_ImplOpenGL3_Init() with "#version 300 es"
#if (defined(__APPLE__) && TARGET_OS_IOS) || (defined(__ANDROID__)) || (defined(__EMSCRIPTEN__))
#define USE_GL_ES3
#endif

I think we should transition to a more explicit #define model like #2351 is using but at the same time do everything we can to default to the right GL header without custom defines.

I'll rework this and try to make a commit soon so don't worry about tweaking the PR now, but would appreciate help on the question above.

Thanks a lot!

[rmitton]

I’m afraid I don’t know which implementations support which version. I’m using PowerVR’s desktop SDK, which definitely only supports 2.0. I was under the impression that Emscripten and whatever Qt includes were also restricted to 2.0, but I can’t confirm that. I’d love to see the correct automatic header detection but I’m afraid I don’t know what the right thing to put in there is.

[ocornut]

Thank you.
I now have merged a similar set of changes (see beb3062, forgot to tag the issue in the commit message)

It is currently using an explicit #define IMGUI_IMPL_OPENGL_ES2.