x11-drivers/xorgxrdp: enable DRI3/glamor support

With the advent of 1.20.x x11-server/xorg-server xorgxrdp can support
dri3/glamor [1] to enable hardware supported OpenGL acceleration with an
xorgxrdp desktop.

Also add pkg-message to explain how to enable hardware acceleration.

While here, fix misuse of "-I" flag to specify LIBDIR.

[1] https://github.com/neutrinolabs/xrdp/issues/1029

PR: 244298
Submitted by: Derek Schrock <dereks@lifeofadishwasher.com> (initial revision)
Approved by: meta (myself, maintainer)
Sponsored by: HAW International