Fix empty wxBitmapComboBox height under wxMSW.

Make (initially) empty wxBitmapComboBox as high as a normal wxComboBox instead
of making it visibly shorter.

Closes #14279.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/branches/WX_3_0_BRANCH@77546 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775
Use bool constants instead of BOOL ones in wxBitmapComboBox code.

Just s/TRUE/true/

See #14279.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/branches/WX_3_0_BRANCH@77547 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775