stbt.Display.frames: Ignore buffers with timestamp == MAX_UNSIGNED_INT
stbt.Display.frames: Ignore buffers with timestamp == MAX_UNSIGNED_INT

Occasionally, immediately after a restart† the HD PVR delivers *one*
frame with a correct timestamp (which sets "self.start_timestamp" here
in "frames") followed by one frame with a timestamp of
18446744073709551615 (2^64 - 1). This caused "frames" to time out
immediately.

"sys.maxsize" is the maximum *signed* integer, i.e. 2^(n-1) - 1.
The maximum unsigned integer is 2^n - 1 == 2 * (2^(n-1) - 1) + 1.

† See commit 5a0770e6.

martynjarvis/stb-tester,martynjarvis/stb-tester,martynjarvis/stb-tester,wmanley/stb-tester,stb-tester/stb-tester,stb-tester/stb-tester,LewisHaley/stb-tester,wmanley/stb-tester,martynjarvis/stb-tester,LewisHaley/stb-tester,wmanley/stb-tester,stb-tester/stb-tester,wmanley/stb-tester,LewisHaley/stb-tester,LewisHaley/stb-tester,LewisHaley/stb-tester,stb-tester/stb-tester,wmanley/stb-tester,LewisHaley/stb-tester,LewisHaley/stb-tester,martynjarvis/stb-tester,martynjarvis/stb-tester,martynjarvis/stb-tester