[qtmozembed] Improve rendering guards

Do not allow refreshing texture in inactive state.
In addition, do not add node to scenegraph if
size is invalid.
Cleanup unnecessary code
Merge pull request #87 from rainemak/improvedRenderingGuard

[qtmozembed] Improve rendering guards
Merge pull request #88 from rainemak/minorCleanup

Cleanup unnecessary code
[unittests] Avoid screen blanking during test runs
[qtmozembed] Fix compiler warning about use of incomplete type