Add an option to embed image size in its name to PNG-to-C script.

Add an option to embed image size in its name to PNG-to-C script.

When a PNG image exists in several sizes it makes sense to use its size as a
suffix to distinguish the different versions, so update the png2c script to
optionally allow to do this.

Current implementation simply checks the PNG header directly to avoid any
extra dependencies.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@66502 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775