mplay32: minor improvement to TogglePlaybackState()

In TogglePlaybackState, replace the if blocks with a switch block and add cases for
MCI_MODE_OPEN and MCI_MODE_STOP. Also include a default case which just
returns. Remove the mode comparision in the window procedure for IDC_PLAY and
IDC_PAUSE and instead always call TogglePlaybackSate. Also lower the scope of
a variable in WM_SIZE.