limit clock precision to 20 us

20 us was the value, firefox used as first response to the spectre attacks.
now it's 100 us or 5 us, depending on whether it's "cross-origin isolated".
we only have one origin, so choosing 20 us is probably fine, I guess
see also:
https://www.mozilla.org/en-US/security/advisories/mfsa2018-01/
https://developer.mozilla.org/en-US/docs/Web/API/Performance/now#security_requirements

other clocks:
* os.time() and os.date() only have seconds precision, AFAIK.
* dtime is only given once per step, so it's not useful
* there might be other ways to build clocks (if we get async envs for sscsm,
  with a busy loop, for example)

src/constants.h

@@ -105,3 +105,7 @@
 // The intent is to ensure that the rendering doesn't turn terribly blurry
 // when filtering is enabled.
 #define TEXTURE_FILTER_MIN_SIZE 192U
+
+// Resolution of clocks that SSCSM has access to, in us.
+// Used as countermeasure against side-channel attacks.
+#define SSCSM_CLOCK_RESOLUTION_US 20