[Public WebGL] getContext multiple context language

[Vladimir Vukicevic]

Here's the language for getContext() -- let me know of any suggestions 
before I send it off to hixie:

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

    'contextId' must be a string naming a canvas rendering context to be
    returned.  This specification only defines one context, with the
    name "2d".  If getContext() is called with that exact string for its
    'contextId' argument, then the UA must return a reference to an
    object implementing CanvasRenderingContext2D.  Other specifications
    may define their own contexts, which would return different objects.

    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.

    If getContext() is called multiple times on the same canvas with the
    same context ID, 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?)

    If getContext() is called with a context ID different from the
    context IDs given to any previous successful calls to getContext(),
    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 multiple rendering contexts are active, they all render to the
    same canvas; they are not layered or otherwise isolated.  Changes
    made to the canvas 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.


- Vlad
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