Optimize FPS calculation using precomputed frame timings

This commit streamlines the FPS calculation within the frame times widget by utilizing the precomputed timings_ms array that already contains the durations between frames in milliseconds. By summing up these precomputed timings, we avoid redundant computation of frame durations from the times_ns timestamps.

The optimized approach calculates the total frame time directly in milliseconds, determines the average frame time, and then computes the FPS by dividing 1000 ms by the average frame time in milliseconds. This change not only improves the efficiency of the code but also simplifies the logic for calculating the average FPS.

Refactoring:

• Use timings_ms instead of recomputing frame durations from times_ns.
• Calculate total_ms and avg_frametime_ms within the FPS update conditional.
• Adjust FPS calculation to 1000.f / avg_frametime_ms.