EMSCRIPTEN: OPENGL: Disable GLAD

configure

@@ -6351,6 +6351,7 @@ if test "$_opengl_mode" != none ; then
 		sdl)
 			# This case is for pure SDL backend. Many platforms overload it and create a new backend
 			# They still make use of SDL but it's handled below in the default case
+			_opengl_glad=yes
 			case $_sdlversion in
 				1.2.*)
 					# Stock SDL 1.2 only supports OpenGL contexts.
@@ -6367,7 +6368,7 @@ if test "$_opengl_mode" != none ; then
 							;;
 						emscripten)
 							_opengl_mode=gles2
-							_opengl_glad=no
+							_opengl_glad=no # https://github.com/Dav1dde/glad-web/issues/12
 							append_var OPENGL_LIBS "-s FULL_ES2=1 -s MAX_WEBGL_VERSION=1"
 							;;
 						*)
@@ -6377,7 +6378,6 @@ if test "$_opengl_mode" != none ; then
 					esac
 					;;
 			esac
-			_opengl_glad=yes
 			;;
 		switch)
 			_opengl_mode=gles2

graphics/opengl/debug.cpp

@@ -39,10 +39,19 @@ Common::String getGLErrStr(GLenum error) {
 		return "GL_INVALID_VALUE";
 	case GL_INVALID_OPERATION:
 		return "GL_INVALID_OPERATION";
+// OpenGL ES 2.0 doesn't have GL_STACK_OVERFLOW and GL_STACK_UNDERFLOW
+// see https://registry.khronos.org/OpenGL/api/GLES2/gl2.h and https://registry.khronos.org/OpenGL/api/GLES2/gl2ext.h
+#ifdef USE_FORCED_GLES2
+	case GL_STACK_OVERFLOW_KHR:
+		return "GL_STACK_OVERFLOW_KHR";
+	case GL_STACK_UNDERFLOW_KHR:
+		return "GL_STACK_UNDERFLOW_KHR";
+#else
 	case GL_STACK_OVERFLOW:
 		return "GL_STACK_OVERFLOW";
 	case GL_STACK_UNDERFLOW:
 		return "GL_STACK_UNDERFLOW";
+#endif
 	case GL_OUT_OF_MEMORY:
 		return "GL_OUT_OF_MEMORY";
 	default:

graphics/opengl/system_headers.h

@@ -84,6 +84,14 @@
 		#define GL_MAX_SAMPLES 0x8D57
 	#endif
 
+	#if !defined(GL_STACK_OVERFLOW_KHR)
+		#define GL_STACK_OVERFLOW_KHR 0x0503
+	#endif
+
+	#if !defined(GL_STACK_UNDERFLOW_KHR)
+		#define GL_STACK_UNDERFLOW_KHR 0x0504
+	#endif
+
 #elif USE_FORCED_GLES
 
 	#define GL_GLEXT_PROTOTYPES