Fix renderer SIGSEGV observed on certain website and profile combinations (#2552)

Rarely, with specific website and profile combinations, canvas can be NULL
inside blink::BaseRenderingContext2D::measureText(), causing a SIGSEGV.

Check for canvas validity before attempting to dereference the canvas
pointer.  Also check for an offscreen render surface and attempt to
apply anti-fingerprinting to that render surface if possible (thanks
to @uazo for the base implementation!)

Tested to work on Debian Bullseye ppc64el

revision.txt

@@ -1 +1 @@
-1
+2