[usdImagingGL] fix thresholds for testUsdImagingGLPurpose

Currently, testUsdImagingGLPurpose relies solely on perceptual diff,
which fails to detect a change between world_0.png (which shows a
sphere) and world_1.png (which is empty). Changing to a reasonable
default for pixel difference (-fail .1, -failpercent 10) which can
successfully distinguish the two images.

pxr/usdImaging/usdImagingGL/CMakeLists.txt

@@ -1854,8 +1854,8 @@ pxr_register_test(testUsdImagingGLPurpose
         world_1.png
         world_2.png
         world_3.png
-    FAIL 1
-    FAIL_PERCENT 1
+    FAIL .1
+    FAIL_PERCENT 10
     PERCEPTUAL
     EXPECTED_RETURN_CODE 0
     TESTENV testUsdImagingGLPurpose