Backends: OpenGL3: Add compatibility of GL_VERSION for GL 2.x

GL_MAJOR_VERSION and GL_MINOR_VERSION are available on GL 3.0 and above.
So we have to parse GL_VERSION under GL 2.x

Regressed since 459de65
See ocornut#3530

backends/imgui_impl_opengl3_loader.h

@@ -685,7 +685,13 @@ static int parse_version(void)
         return GL3W_ERROR_INIT;
     glGetIntegerv(GL_MAJOR_VERSION, &version.major);
     glGetIntegerv(GL_MINOR_VERSION, &version.minor);
-    if (version.major < 3)
+    if (version.major == 0 && version.minor == 0)
+    {
+        // Query GL_VERSION in desktop GL 2.x, the string will start with "<major>.<minor>"
+        const char* gl_version = (const char*)glGetString(GL_VERSION);
+        sscanf(gl_version, "%d.%d", &version.major, &version.minor);
+    }
+    if (version.major < 2)
         return GL3W_ERROR_OPENGL_VERSION;
     return GL3W_OK;
 }