Support additional pixdepths other than 32 in XvfbConfig

[mike10004]

XvfbConfig currently only supports 32 as an additional pixdepth in the geometry. This should support any value from 1 to 32, as specified in the Xvfb documentation.

The violating code is:

public XvfbConfig(String geometry) {
    this.geometry = checkNotNull(geometry);
    checkArgument(geometry.matches("\\d+x\\d+x\\d+(?:\\+32)?"), "argument must have form WxHxD where W=width, H=height, and D=depth; default is 1280x1024x24+32");
}