SampleApp builds skia on Linux with BGRA pixel order, but Ganesh's SampleApp

SampleApp builds skia on Linux with BGRA pixel order, but Ganesh's SampleApp
assumes GL_RGBA texture format on all platforms except Windows (where it uses
GL_BGRA), so texture colours come out wrong.  As a temporary fix, do on Linux
what we do on Windows, and assume GL_BGRA.  (This should be made more general
later, and the assumption of matching formats between Skia and Ganesh removed.)

Review URL:  http://codereview.appspot.com/4474046/

git-svn-id: http://skia.googlecode.com/svn/trunk@1247 2bbb7eff-a529-9590-31e7-b0007b416f81