GL: list WEBGL_debug_renderer_info in the startup log.

Because its usage is rather specific, there wasn't no
extension-dependent dispatch happening and thus the extension was never
marked as used. But that was confusing since it actually is, so add an
explicit branch just to list the extension.

src/Magnum/GL/Implementation/ContextState.cpp

@@ -28,21 +28,44 @@
 #include <Corrade/Containers/StringView.h>
 
 #include "Magnum/GL/Context.h"
+#include "Magnum/GL/Extensions.h"
 
 namespace Magnum { namespace GL { namespace Implementation {
 
 using namespace Containers::Literals;
 
-ContextState::ContextState(Context& context, Containers::StaticArrayView<Implementation::ExtensionCount, const char*>) {
+ContextState::ContextState(Context& context, const Containers::StaticArrayView<Implementation::ExtensionCount, const char*> extensions) {
     #ifndef MAGNUM_TARGET_GLES
     if((context.detectedDriver() & Context::DetectedDriver::NVidia) &&
         !context.isDriverWorkaroundDisabled("nv-zero-context-profile-mask"_s))
     {
         isCoreProfileImplementation = &Context::isCoreProfileImplementationNV;
     } else isCoreProfileImplementation = &Context::isCoreProfileImplementationDefault;
-    #else
+    #endif
+
+    #ifdef MAGNUM_TARGET_WEBGL
+    /* The rendererStringUnmasked() and vendorStringUnmasked() branch on the
+       extension on their own, which is in-line with all other "limit" queries.
+       It wouldn't make sense to create four new *Implementation()
+       functions and two new pointers for something that gets called mostly
+       just on application startup (and where it can't actually use the
+       function pointer because at that point the state is still yet to be
+       created).
+
+       But since there's nothing else would add the extension to the used
+       extension list, we're doing that here. */
+    if(context.isExtensionSupported<Extensions::WEBGL::debug_renderer_info>()) {
+        extensions[Extensions::WEBGL::debug_renderer_info::Index] =
+                   Extensions::WEBGL::debug_renderer_info::string();
+    }
+    #endif
+
+    #if defined(MAGNUM_TARGET_GLES) && !defined(MAGNUM_TARGET_WEBGL)
     static_cast<void>(context);
     #endif
+    #ifndef MAGNUM_TARGET_WEBGL
+    static_cast<void>(extensions);
+    #endif
 }
 
 }}}