[libretro] Reverting back to the use of clock()

clock() was previously replaced by frame time callback (1c8eb84 #2589)
It introduced a few bugs that I tried to fix (#2829)
But there is a [benchmark cartridge](https://tic80.com/play?cart=153)
that calls time() to mesure itra-frame timing.

This is how Gemini explains it:

The situation involves two different timing requirements:

Inter-Frame Timing (Game Speed): This is about ensuring the game
logic advances correctly from one frame to the next. If 1/60th of a
second has passed in the real world, the game should also advance its
state by 1/60th of a second. The original fix (`state->frameTime +=
usec;`) solved this by correctly accumulating the time between frames.

Intra-Frame Timing (Benchmark Measurement): This is what the
benchmark cartridge needs. It uses TIC-80's `time()` function to measure
how long an operation takes within a single frame. It records the start
time, does a lot of drawing, records the end time, and calculates the
difference.

The original fix failed the benchmark test because the
`state->frameTime` value is only updated once per frame. For the entire
duration of the `MAINTIC()` function, the value returned by `time()`
would be constant. Therefore, `time() - stime` would always be zero.

--
The clock() solution is superior because it provides a high-resolution,
continuously updating timer directly from the system's C library.

clock(): This standard function returns the amount of processor time
used by the program. Crucially, this value increases during the
execution of a single frame.

CLOCKS_PER_SEC: This constant provides the frequency of the clock()
timer.

When the benchmark calls time() (`clock() / CLOCKS_PER_SEC`), it gets a
precise timestamp. After it finishes its work and calls time() again,
the value from clock() will have increased, yielding a correct, non-zero
`runningTime`. This solves the intra-frame timing problem.

Simultaneously, because clock() is a consistently increasing counter,
the TIC-80 engine can use it to perfectly measure the duration between
frames, solving the inter-frame timing problem as well.

Author: imsys

src/system/libretro/tic80_libretro.c

@@ -77,7 +77,6 @@ struct tic80_state
 	int mouseHideTimer;
 	int mouseHideTimerStart;
 	tic80* tic;
-	retro_usec_t frameTime;
 };
 static struct tic80_state* state = NULL;
 
@@ -86,19 +85,15 @@ static struct tic80_state* state = NULL;
  */
 static u64 tic80_libretro_counter()
 {
-	if (state == NULL) {
-		return 0;
-	}
-
-	return (u64)state->frameTime;
+	return clock();
 }
 
 /**
  * TIC-80 callback; Request frequency.
  */
 static u64 tic80_libretro_frequency()
 {
-	return TIC80_FREQUENCY;
+	return CLOCKS_PER_SEC;
 }
 
 /**
@@ -155,17 +150,6 @@ void tic80_libretro_fallback_log(enum retro_log_level level, const char *fmt, ..
 	va_end(va);
 }
 
-/**
- * libretro callback; Called to indicate how much time has passed since last retro_run().
- */
-void tic80_libretro_frame_time(retro_usec_t usec) {
-	if (state == NULL) {
-		return;
-	}
-
-	state->frameTime += usec;
-}
-
 /**
  * libretro callback; Global initialization.
  */
@@ -1116,16 +1100,6 @@ RETRO_API bool retro_load_game(const struct retro_game_info *info)
 		return false;
 	}
 
-	// Set up the frame time callback.
-	struct retro_frame_time_callback frame_time = {
-		.callback = tic80_libretro_frame_time,
-		.reference = TIC80_FREQUENCY / TIC80_FRAMERATE,
-	};
-	if (!environ_cb(RETRO_ENVIRONMENT_SET_FRAME_TIME_CALLBACK, &frame_time)) {
-		log_cb(RETRO_LOG_ERROR, "[TIC-80] Failed to set frame time callback.\n");
-		return false;
-	}
-
 	// Set up the TIC-80 environment.
 #if RETRO_IS_BIG_ENDIAN
 	state->tic = tic80_create(TIC80_SAMPLERATE, TIC80_PIXEL_COLOR_ARGB8888);