[trunk] PNG resolution preference ignored

Steps to reproduce bug
1. Edit > Preferences... > Export tab
2. Change "Resolution DPI"
3. File > Save As...
4. Choose PNG as file format in the save dialog

Expected behavior: PNG should export at the DPI specified in the Preference dialog

Actual behavior: PNG always exports at 300 DPI regardless of Preference setting

MuseScore version: r. 3575 nightly trunk

(Operating System: Windows 7)