Backends: OpenGL3: Fixed loader compatibility with GL_VERSION for GL 2.x (#6154, #4445, #3530)

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
Reference https://www.khronos.org/opengl/wiki/OpenGL_Context#Context_information_queries
Regressed since 459de65477
See https://github.com/ocornut/imgui/pull/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;
 }
@@ -709,7 +715,7 @@ int imgl3wInit2(GL3WGetProcAddressProc proc)
 
 int imgl3wIsSupported(int major, int minor)
 {
-    if (major < 3)
+    if (major < 2)
         return 0;
     if (version.major == major)
         return version.minor >= minor;

docs/CHANGELOG.txt

@@ -116,7 +116,8 @@ All changes:
 - Backends: GLFW: Added ImGui_ImplGlfw_SetCallbacksChainForAllWindows() to instruct backend
   to chain callbacks even for secondary viewports/windows. User callbacks may need to test
   the 'window' parameter. (#6142)
-- Backends: WebGPU: Fix building for latest WebGPU specs (remove implicit layout generation).
+- Backends: OpenGL3: Fixed GL loader compatibility with 2.x profiles. (#6154, #4445, #3530) [@grauw]
+- Backends: WebGPU: Fixed building for latest WebGPU specs (remove implicit layout generation).
   (#6117, #4116, #3632) [@tonygrue, @bfierz]
 - Examples: refactored SDL2+GL and GLFW+GL examples to compile with Emscripten.
   (#2492, #2494, #3699, #3705) [@ocornut, @nicolasnoble]