Add wxDataViewRendererBase::GetEffectiveAlignment() and use it.

Add wxDataViewRendererBase::GetEffectiveAlignment() and use it.

This helper method falls back on the alignment of the column if the renderer
alignment is not specified. This is almost always what should be used instead
of GetAlignment() to determine the alignment that really should be used in the
drawing code.

In particular, using GetEffectiveAlignment() in wxDataViewCustomRenderer fixes
the problem with bitmap columns ignoring column alignment for their bitmaps.

Closes #15498.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@74825 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775