DirectShow: Fix MinGW warnings about signedness of comparison.

Use a switch on int instead of else if.

common\evr\evrcustompresenter.cpp: In member function 'virtual bool EVRCustomPresenter::event(QEvent*)':
common\evr\evrcustompresenter.cpp:1882:22: warning: comparison between 'enum QEvent::Type' and 'enum EVRCustomPresenter::PresenterEvents' [-Wenum-compare]
     if (e->type() == StartSurface) {
                      ^
common\evr\evrcustompresenter.cpp:1885:29: warning: comparison between 'enum QEvent::Type' and 'enum EVRCustomPresenter::PresenterEvents' [-Wenum-compare]
     } else if (e->type() == StopSurface) {
                             ^
common\evr\evrcustompresenter.cpp:1888:29: warning: comparison between 'enum QEvent::Type' and 'enum EVRCustomPresenter::PresenterEvents' [-Wenum-compare]
     } else if (e->type() == PresentSample) {

Change-Id: I8533e2c6d9b0a01b7b95e1d8bb119b50d4aabd25
Reviewed-by: Mark Brand <mabrand@mabrand.nl>