DebugTools: use steady_clock for FrameProfiler CPU timing.

Otherwise it may happen that the clock gets adjusted mid-frame, leading
to an underflow and an assertion like this:

    Assertion _measurements[i]._movingSum + data >= _measurements[i]._movingSum failed at src/Magnum/DebugTools/FrameProfiler.cpp:233

The clock adjustment is known to be happening rather frequently under
WSL2.

src/Magnum/DebugTools/FrameProfiler.cpp

@@ -476,7 +476,11 @@ void FrameProfilerGL::setup(const Values values, const UnsignedInt maxFrameCount
         arrayAppend(measurements, InPlaceInit,
             "Frame time"_s, Units::Nanoseconds, UnsignedInt(Containers::arraySize(_state->frameTimeStartFrame)),
             [](void* state, UnsignedInt current) {
-                static_cast<State*>(state)->frameTimeStartFrame[current] = std::chrono::duration_cast<std::chrono::nanoseconds>(std::chrono::high_resolution_clock::now().time_since_epoch()).count();
+                /* Using steady_clock even though it might not be as precise as
+                   high_resolution_clock in order to avoid the delta being
+                   negative between frames, causing an underflow and an assert
+                   similar to what used to happen for PrimitiveClipRatio */
+                static_cast<State*>(state)->frameTimeStartFrame[current] = std::chrono::duration_cast<std::chrono::nanoseconds>(std::chrono::steady_clock::now().time_since_epoch()).count();
             },
             [](void*, UnsignedInt) {},
             [](void* state, UnsignedInt previous, UnsignedInt current) {
@@ -490,11 +494,15 @@ void FrameProfilerGL::setup(const Values values, const UnsignedInt maxFrameCount
         arrayAppend(measurements, InPlaceInit,
             "CPU duration"_s, Units::Nanoseconds,
             [](void* state) {
-                static_cast<State*>(state)->cpuDurationStartFrame = std::chrono::duration_cast<std::chrono::nanoseconds>(std::chrono::high_resolution_clock::now().time_since_epoch()).count();
+                /* Using steady_clock even though it might not be as precise as
+                   high_resolution_clock in order to avoid the delta being
+                   negative between frames, causing an underflow and an assert
+                   similar to what used to happen for PrimitiveClipRatio */
+                static_cast<State*>(state)->cpuDurationStartFrame = std::chrono::duration_cast<std::chrono::nanoseconds>(std::chrono::steady_clock::now().time_since_epoch()).count();
             },
             [](void* state) {
                 /* libc++ 10 needs an explicit cast to UnsignedLong */
-                return UnsignedLong(std::chrono::duration_cast<std::chrono::nanoseconds>(std::chrono::high_resolution_clock::now().time_since_epoch()).count() - static_cast<State*>(state)->cpuDurationStartFrame);
+                return UnsignedLong(std::chrono::duration_cast<std::chrono::nanoseconds>(std::chrono::steady_clock::now().time_since_epoch()).count() - static_cast<State*>(state)->cpuDurationStartFrame);
             }, _state.get());
         _state->cpuDurationIndex = index++;
     }