This can be used to test if a layout if possible.

Better to avoid making unnecessary calls, but mainly we want to avoid
logging false errors.

For example, if we have earlier disabled an output because it was too
large for the framebuffer, we should prefer this output when
allocating new outputs.

Move the code that turn off unused outputs to the end.

Also, add support for checking the output connection state.

Apparently this is not necessary when using the internal API, but a
must with libXrandr.

Return error if no CRTC.

Screen is a Xvnc thing and not relevant for x0vncserver etc.

In RandR land, there's a lot of return code confusion. Our wrappers
are using the same return codes as RRCrtcSet, RRScreenSizeSet: 1/TRUE
for success. Fixes:

* vncRandRCreateOutputs did not follow this convention

* A lot of code returned -1 upon failure

* vncRandRDisableOutput returned 0 for already disabled outputs

These two code blocks are not specific to Xvnc/vnc.so, but useful for
x0vncserver as well. RandrGlue.h defines the interface on which
unixcommon depends on.