egl: implement SDL_GL_EGL_PLATFORM attribute

This implements a new SDL_GL_EGL_PLATFORM attribute to set the
"platform" argument for SDL_EGL_LoadLibrary on Windows, macOS, and
Linux. I've limited it to those three operating systems because that's
what I've been able to test against.

include/SDL_video.h

@@ -249,7 +249,8 @@ typedef enum
     SDL_GL_CONTEXT_RELEASE_BEHAVIOR,
     SDL_GL_CONTEXT_RESET_NOTIFICATION,
     SDL_GL_CONTEXT_NO_ERROR,
-    SDL_GL_FLOATBUFFERS
+    SDL_GL_FLOATBUFFERS,
+    SDL_GL_EGL_PLATFORM
 } SDL_GLattr;
 
 typedef enum

src/video/SDL_sysvideo.h

@@ -391,6 +391,7 @@ struct SDL_VideoDevice
         int framebuffer_srgb_capable;
         int no_error;
         int retained_backing;
+        int egl_platform;
         int driver_loaded;
         char driver_path[256];
         void *dll_handle;

src/video/SDL_video.c

@@ -3673,6 +3673,8 @@ SDL_GL_ResetAttributes()
     _this->gl_config.reset_notification = SDL_GL_CONTEXT_RESET_NO_NOTIFICATION;
 
     _this->gl_config.share_with_current_context = 0;
+
+    _this->gl_config.egl_platform = 0;
 }
 
 int
@@ -3789,6 +3791,9 @@ SDL_GL_SetAttribute(SDL_GLattr attr, int value)
     case SDL_GL_CONTEXT_NO_ERROR:
         _this->gl_config.no_error = value;
         break;
+    case SDL_GL_EGL_PLATFORM:
+        _this->gl_config.egl_platform = value;
+        break;
     default:
         retval = SDL_SetError("Unknown OpenGL attribute");
         break;
@@ -3998,6 +4003,12 @@ SDL_GL_GetAttribute(SDL_GLattr attr, int *value)
             *value = _this->gl_config.no_error;
             return 0;
         }
+    case SDL_GL_EGL_PLATFORM:
+        {
+            *value = _this->gl_config.egl_platform;
+            return 0;
+        }
+        break;
     default:
         return SDL_SetError("Unknown OpenGL attribute");
     }

src/video/cocoa/SDL_cocoaopengles.m

@@ -51,7 +51,7 @@ Cocoa_GLES_LoadLibrary(_THIS, const char *path)
     }
     
     if (_this->egl_data == NULL) {
-        return SDL_EGL_LoadLibrary(_this, NULL, EGL_DEFAULT_DISPLAY, 0);
+        return SDL_EGL_LoadLibrary(_this, NULL, EGL_DEFAULT_DISPLAY, _this->gl_config.egl_platform);
     }
 
     return 0;
@@ -127,7 +127,7 @@ Cocoa_GLES_SetupWindow(_THIS, SDL_Window * window)
         #if 0  /* When hint SDL_HINT_OPENGL_ES_DRIVER is set to "1" (e.g. for ANGLE support), _this->gl_config.driver_loaded can be 1, while the below lines function. */
         SDL_assert(!_this->gl_config.driver_loaded);
         #endif
-        if (SDL_EGL_LoadLibrary(_this, NULL, EGL_DEFAULT_DISPLAY, 0) < 0) {
+        if (SDL_EGL_LoadLibrary(_this, NULL, EGL_DEFAULT_DISPLAY, _this->gl_config.egl_platform) < 0) {
             SDL_EGL_UnloadLibrary(_this);
             return -1;
         }

src/video/windows/SDL_windowsopengles.c

@@ -52,7 +52,7 @@ WIN_GLES_LoadLibrary(_THIS, const char *path) {
     }
     
     if (_this->egl_data == NULL) {
-        return SDL_EGL_LoadLibrary(_this, NULL, EGL_DEFAULT_DISPLAY, 0);
+        return SDL_EGL_LoadLibrary(_this, NULL, EGL_DEFAULT_DISPLAY, _this->gl_config.egl_platform);
     }
 
     return 0;
@@ -113,7 +113,7 @@ WIN_GLES_SetupWindow(_THIS, SDL_Window * window)
         #if 0  /* When hint SDL_HINT_OPENGL_ES_DRIVER is set to "1" (e.g. for ANGLE support), _this->gl_config.driver_loaded can be 1, while the below lines function. */
         SDL_assert(!_this->gl_config.driver_loaded);
         #endif
-        if (SDL_EGL_LoadLibrary(_this, NULL, EGL_DEFAULT_DISPLAY, 0) < 0) {
+        if (SDL_EGL_LoadLibrary(_this, NULL, EGL_DEFAULT_DISPLAY, _this->gl_config.egl_platform) < 0) {
             SDL_EGL_UnloadLibrary(_this);
             return -1;
         }

src/video/x11/SDL_x11opengles.c

@@ -55,7 +55,7 @@ X11_GLES_LoadLibrary(_THIS, const char *path)
         #endif
     }
     
-    return SDL_EGL_LoadLibrary(_this, path, (NativeDisplayType) data->display, 0);
+    return SDL_EGL_LoadLibrary(_this, path, (NativeDisplayType) data->display, _this->gl_config.egl_platform);
 }
 
 XVisualInfo *