Turn off multisampling in two tests.

TestLogoWidgetDepthPeeling, TestLogoWidgetAlphaBlending.
These two tests look like they were trying to disable multisampling
by calling SetMultiSamples(1), where VTK's default number of
multisamples is 8.  However, on at least one system (OS X 10.5 with
Radeon 9600 XT) these tests fail unless the sample buffer is disabled
completely.

Change-Id: I287933920cd78d02b34ab44a2d765deebd7d6b3a
Merge topic 'osx-multi-samples' into master

4ecfc7f3 Turn off multisampling in two tests.