nv2a: Ptimer fixes

Ported from https://github.com/xqemu/xqemu/pull/113 by dracc https://github.com/dracc

A few GPU timing related fixes
NV2A crystal frequency is currently set to debug unit speed of 13.5MHz.
Later retail xboxes had a 16.67MHz crystal according to @JayFoxRox's research(?).
Also, current ptimer clock value calculation is flipped around and on top of that always overflows.

With these fixes, running get_timers.py against xqemu reports values close to what real hardware gives.

Proposed changes
Set NV2A_CRYSTAL_FREQ to retail speed
Fix ptimer_get_clock() behaviour

src/devices/video/EmuNV2A_PTIMER.cpp

@@ -41,15 +41,19 @@
 
 #include "CxbxCommon.h"
 
+#define NANOSECONDS_PER_SECOND 1000000000
+
 /* PTIMER - time measurement and time-based alarms */
 static uint64_t ptimer_get_clock(NV2AState * d)
 {
 	// Get time in nanoseconds
     uint64_t time = std::chrono::duration_cast<std::chrono::nanoseconds>(std::chrono::high_resolution_clock::now().time_since_epoch()).count();
 	
-	return Muldiv64(time,
-					uint32_t(d->pramdac.core_clock_freq * d->ptimer.numerator),
-					CLOCKS_PER_SEC * d->ptimer.denominator);
+	return Muldiv64(Muldiv64(time,
+					d->pramdac.core_clock_freq,
+					NANOSECONDS_PER_SECOND), // Was CLOCKS_PER_SEC
+				d->ptimer.denominator,
+				d->ptimer.numerator);
 }
 
 DEVICE_READ32(PTIMER)

src/devices/video/nv2a_int.h

@@ -1753,7 +1753,7 @@
 #define NV2A_VERTEX_ATTR_RESERVED2      14
 #define NV2A_VERTEX_ATTR_RESERVED3      15
 
-#define NV2A_CRYSTAL_FREQ 13500000
+#define NV2A_CRYSTAL_FREQ 16666666 // Was 13500000
 #define NV2A_NUM_CHANNELS 32
 #define NV2A_NUM_SUBCHANNELS 8