fix OpenGL version detection

This allows OpenGL waveforms to be used with the Wayland Qt
Platform Abstraction on KDE and Weston. Though GL_MAJOR_VERSION and
GL_MINOR_VERSION reported much higher versions (4.6 when tested on
Intel UHD Graphics 770 (ADL-S GT1)), QSurfaceFormat reports 2.0 on
KDE Wayland and Weston (somehow it reports 2.1 on GNOME Wayland
though) and 2.1 with the Xcb QPA. QSurfaceFormat does not report
higher versions if they are requested in the OpenGLWindow
constructor.

src/waveform/waveformwidgetfactory.cpp

@@ -142,13 +142,29 @@ WaveformWidgetFactory::WaveformWidgetFactory()
                     reinterpret_cast<const char*>(glFunctions->glGetString(GL_VENDOR))));
             QString rendererString = QString(QLatin1String(
                     reinterpret_cast<const char*>(glFunctions->glGetString(GL_RENDERER))));
-            qDebug() << versionString << vendorString << rendererString;
-
-            // note: the requested version has been set in WGLWidget's OpenGLWindow constructor
-            const int majorVersion = context->surface()->format().majorVersion();
-            const int minorVersion = context->surface()->format().minorVersion();
+            qDebug().noquote() << QStringLiteral(
+                    "OpenGL driver version string \"%1\", vendor \"%2\", "
+                    "renderer \"%3\"")
+                                          .arg(versionString, vendorString, rendererString);
+
+            GLint majorVersion = -1;
+            glFunctions->glGetIntegerv(GL_MAJOR_VERSION, &majorVersion);
+            GLint minorVersion = -1;
+            glFunctions->glGetIntegerv(GL_MINOR_VERSION, &minorVersion);
+            if (majorVersion == -1 || minorVersion == -1) {
+                // QSurfaceFormat may underreport the supported version (at
+                // least with the Wayland and Xcb Qt Platform Abstractions), so
+                // prefer querying GL_MAJOR/MINOR_VERSION.
+                // GL_MAJOR/MINOR_VERSION are not supported below OpenGL 3.0, so
+                // use what QSurfaceFormat reports as a fallback.
+                // https://www.khronos.org/opengl/wiki/OpenGL_Context#OpenGL_version_number
+                majorVersion = context->surface()->format().majorVersion();
+                minorVersion = context->surface()->format().minorVersion();
+            }
 
-            qDebug() << "QOpenGLContext surface format version:" << majorVersion << minorVersion;
+            qDebug().noquote()
+                    << QStringLiteral("Supported OpenGL version: %1.%2")
+                               .arg(QString::number(majorVersion), QString::number(minorVersion));
 
             m_openGLShaderAvailable = QOpenGLShaderProgram::hasOpenGLShaderPrograms(context);