No changes, just refactor wxCairoBitmapData ctor.

No changes, just refactor wxCairoBitmapData ctor.

Extract the code for buffer and surface creation into separate helper
functions so that they could be reused by other constructors (to be added in
the next commits).

Also use cairo_format_stride_for_width(), if available, to compute the stride
for the surfaces we create instead of hard coding 4*width.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@69352 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775