Add EGL+OpenGLES 3.0 examples

[guangli0510]

Signed-off-by: Guangli Li guangli.li@intel.com

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[ocornut]

Hello @guangli0510. Thanks for your PR.

I don't quite understand the approach you used - you completely duplicated imgui_impl_opengl3.cpp and made no sensible change to it (the only change being a default GLSL version that can already be setup through the initialization parameter, and which you did setup in main.cpp). Therefore the entirely of the new imgui_impl_opengles3.cpp file is unnecessary.

More over, none of the description/commentary were updated.

It looks like the change should be:

• update gl3w to be GL friendly
• update the existing examples/example_glfw_opengl3 to support a define that would use GL ES 3 and remove all the code duplication
• update comments/documentation
• Is __EGL__ a standard define for the purpose of selecting GL ES? If there a possibly more standard define to use?

The good news is that the simplicity of this change suggests that the recent changes to support shader variations (applied in #1938) are indeed allowing GL ES 3 to work.

[guangli0510]

Hello @ocornut ,
I only add an example of EGL and OpenGL ES. Because we are working on linux and want to communicate with android app. As it is known, android uses EGL+OpenGL ES, but I don't find any examples in imGUI folder, so I complete this usage and submit this to your git.

1. EGL only defines to choose EGL and OpenGL ES, not just for OpenGL ES. In EGL version, it should use eglGetProcAddress to get proc address, not use glXGetProcAddress.
2. I install egl and GLES library, and only find GLES version 100 is runnable for OpenGL ES3.0, so I change the GLSL version to 100.

The patch comment is as below:
diff:

+#ifdef __EGL__
+#include <stdio.h>
+#include <EGL/egl.h>  // this is for eglGetProcAddress
+#include <stdlib.h>
+#else
 #include <GL/glx.h>  //just use opengl API to get proc address.
+#endif
 
-static void *libgl;
+static void *libgl = NULL;
 
 static void open_libgl(void)
 {
-       libgl = dlopen("libGL.so.1", RTLD_LAZY | RTLD_GLOBAL);
+#ifdef __EGL__   //use libGLESv2.so to get all the APIs proc
+       libgl = dlopen("libGLESv2.so", RTLD_LAZY | RTLD_GLOBAL);
+    if(NULL == libgl) {
+           fprintf(stderr, "open libGLESv2.so failed\n");
+    }
+#else  //default
+    libgl = dlopen("libGL.so.1", RTLD_LAZY | RTLD_GLOBAL);
+    if(NULL == libgl) {
+        fprintf(stderr, "open libGL.so failed\n");
+    }
+#endif
 }


-       void *res;
-       res = (void*)glXGetProcAddress((const GLubyte *) proc);
+       void *res = NULL;
+#ifdef __EGL__  // use eglGetProcAddress  to get API proc
+       res = eglGetProcAddress(proc);
+#else
+    res = (void*)glXGetProcAddress((const GLubyte *) proc);
+#endif