stress-test: Avoid overflows in clip rectangles

stress-test: Avoid overflows in clip rectangles

The rectangles in the clip region set in set_general_properties()
would sometimes overflow, which would lead to messages like these:

      *** BUG ***
      In pixman_region32_union_rect: Invalid rectangle passed
      Set a breakpoint on '_pixman_log_error' to debug

when the micro version number of pixman is even.

Fix this by detecting the overflow and clamping such that the x2/y2
coordinates are less than INT32_MAX.