coyote: CGKML_SCREENOVERLAY__DEFINE

Description
This program implements the KML ScreenOverlay element. For 
reference, see  the `Google KML Reference Documentation `.
A ScreenOverlay element draws an image onto the Google Earth display or screen.
Categories
Graphics, FileIO
Examples
See the `cgKML_File` object for examples of how to create a KML file.
Author
FANNING SOFTWARE CONSULTING::
   David W. Fanning 
   1645 Sheely Drive
   Fort Collins, CO 80526 USA
   Phone: 970-221-0438
   E-mail: david@idlcoyote.com
   Coyote's Guide to IDL Programming: http://www.idlcoyote.com
History
Change History::
   Written, 3 November 2012 by David W. Fanning.
Copyright
Copyright (c) 2012, Fanning Software Consulting, Inc.
e purpose of this method is to establish an abstract KML ScreenOverlay class. For reference, see
tps://developers.google.com/kml/documentation/kmlreference?hl=fr#ScreenOverlay.
Keywords
overlay_xy: in, required, type=dblarr
    This keyword specifies a point [x,y] in the image overlay that is mapped to the 
    screen coordinate `Screen_XY`. The location of the point is specified by the values 
    in `Overlay_Units_X` and 'Overlay_Units_Y`. 
overlay_unit_x: in, optional, type=string, default='fraction'
    Three values are possible: (1) "fraction" indicates that the X value is a fraction of the image;
    (2) "pixels" indicates the the X value is in pixels; and (3) "insetPixels" indicates the X
    value is an indent from the right edge of the image.
overlay_unit_y: in, optional, type=string, default='fraction'
    Three values are possible: (1) "fraction" indicates that the Y value is a fraction of the image;
    (2) "pixels" indicates the the Y value is in pixels; and (3) "insetPixels" indicates the Y
    value is an indent from the top edge of the image.
rotation: in, optional, type=double, default=0.0
    This keyword indicates the angle of rotation of the screen overlay in degrees counterclockwise
    from North. A value from -180 to 180.
screen_xy: in, optional, type=dblarr
    This keyword specifies a point [x,y] relative to the screen origin that the overlay image 
    is mapped to. The location of the point is specified by the values in `XUnits` and 'YUnits`. 
screen_unit_x: in, optional, type=string, default='fraction'
    Three values are possible: (1) "fraction" indicates that the X value is a fraction of the image;
    (2) "pixels" indicates the the X value is in pixels; and (3) "insetPixels" indicates the X
    value is an indent from the right edge of the image.
screen_unit_y: in, optional, type=string, default='fraction'
    Three values are possible: (1) "fraction" indicates that the Y value is a fraction of the image;
    (2) "pixels" indicates the the Y value is in pixels; and (3) "insetPixels" indicates the Y
    value is an indent from the top edge of the image.
size_xy: in, required, type=dblarr
    This keyword specifies the size of the image for the screen overlay as follows. A -1
    indicates to use the native dimensions of the image. A 0 indicates that the aspect ratio
    of the image should be preserved. Any other value sets the output dimension of the image,
    according to the "unit" values.
size_unit_x: in, optional, type=string, default='fraction'
    Three values are possible: (1) "fraction" indicates that the X value is a fraction of the image;
    (2) "pixels" indicates the the X value is in pixels; and (3) "insetPixels" indicates the X
    value is an indent from the right edge of the image.
size_unit_y: in, optional, type=string, default='fraction'
    Three values are possible: (1) "fraction" indicates that the Y value is a fraction of the image;
    (2) "pixels" indicates the the Y value is in pixels; and (3) "insetPixels" indicates the Y
    value is an indent from the top edge of the image. 
_ref_extra: out, optional
    Any keywords for the superclass objects are allowed.
e purpose of this method is to destroy anything we created that may leak
mory or need to be released from use.
e cgKML_ScreenOverlay class definition module. It is a container object and 
presents part of the KML file hierarchy.
Params
class: out, optional, type=structure
    The class definition returned as a structure variable. Occassionally useful.