Fix rendering/point-size conformance test

Add a missing gl.clear in rendering/point-size that makes this test passable.

Currently the test always fails, because it does not clear the frame buffer between rendering passes. It clears, draws a point of size 1, clears, draws a point of size 2, then immediately will draw a point of size 1 without clearing, then finish with a clear and a point of size 2. Thus the second point of size 1 is detected as a point of size 2.

This pull request relocates the clear so it gets called on every iteration of test().