[Public WebGL] getContext multiple context language

[Vladimir Vukicevic]

Ok, here's some updated text... how's this look?

    object getContext(in DOMString contextId, in optional any attributes)

    'contextId' must be a string naming a canvas rendering context to be
    returned.  For example, this specification defines the '2d' context,
    which, if requested, will return either a reference to an object
    implementing CanvasRenderingContext2D or null, if a 2D context
    cannot be created at this time.  Other specifications may define
    their own contexts, which would return different objects.

    If getContext() is called with a contextId that the implementation
    supports and such a context can be created, a reference to an object
    implementing the context API is returned and the new context is
    added to the list of active contexts for this canvas.  If contextId
    is not supported or it cannot be created, null is returned.  The
    optional 'attributes' parameter must be either null, or an object
    specific to the context being requested.  A null or unspecified
    value for the 'attributes' parameter is equivalent to requesting
    default attributes for the given context.  Any unknown attributes
    should be ignored by the context.

    If the canvas has at least one active context and getContext() is
    called with a contextId that names an already-active context, it
    must return the same resulting context object provided that the
    attributes are identical to all previous requests for that context
    ID.  If the attributes are not identical, getContext() must raise an
    INVALID_STATE_ERR exception.  (XXX is INVALID_STATE the right thing
    here? or is SYNTAX_ERR better?)  Deciding which attributes are
    identical is up to the specific context being requested, but should
    mean that the requested attributes and their values are the same as
    in all previous getContext() calls with this contextId.

    If the canvas has at least one active context and getContext() is
    called with a contextId that does not name an already-active
    context, it may either return a new canvas context that can be used
    simultaneously with all previously obtained contexts, or return null
    to indicate that the newly requested context cannot be used with the
    previously obtained contexts.  If a non-null value is returned, the
    new context is added to the list of active contexts for this canvas.

    If multiple rendering contexts are active, they all render to the
    same canvas bitmap; they are not layered or otherwise isolated. 
    Changes made to the canvas bitmap with one context must be
    immediately visible to any other active contexts on the canvas.  It
    is up to the implementation to manage synchronization issues
    associated with rendering with different contexts to the same canvas.


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