Rewrite ConvertToGreyscale() and ConvertToDisabled()
They do almost the same thing, so use almost the same code.
Fixes ConvertToDisabled() modifying its own data.
Closes #15389

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@74625 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775
Signed-off-by: Marian 'VooDooMan' Meravy <vdm113@gmail.com>

Conflicts:
	src/common/image.cpp
use ICC 13 for patcher, instead of ICC 14 since it is in beta phase

Signed-off-by: Marian 'VooDooMan' Meravy <vdm113@gmail.com>
patched for ICC

Signed-off-by: Marian 'VooDooMan' Meravy <vdm113@gmail.com>