add LoadIcon*

[tmm1]

These methods allow loading smaller and larger icon sizes instead of the default size.

Note that these methods require Windows Vista or later. Is there a pattern in-place for loading these functions gracefully (instead of via MustGetProcAddress)?

cc lxn/walk#336

[lxn]

Probably just load them like this:

drawThemeTextEx, _ = syscall.GetProcAddress(syscall.Handle(libuxtheme), "DrawThemeTextEx")