Improve appearance of text with background colour in wxHTML.

Use wxHTML_CLR_TRANSPARENT_BACKGROUND instead of wxHTML_CLR_BACKGROUND for the
bgcolor attribute of the body tag to prevent clipping of the text cells by the
next cell with opaque background.

Closes #14986.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@73412 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775