canvas updates in response to chairs request for clarification on issue 131

This modification includes:

- a clarification on the text baseline returned
- modifying getCaretBlinkRate to return a WebIDL defined long value and
ensuring any return value <0 indicates that the OS platform does not
support a caret blink rate setting

Also, I want to point people to a modification to the 11239 bug report by
Charles Pritchard that points to browser source code that is in WebKit that
can be reused for text baseline computation on 4/13
http://www.w3.org/Bugs/Public/show_bug.cgi?id=11239  This way browser
vendors can go to the code and easily make a patch for the baseline
modification.

I am updating (in the next half hour) the change proposal for Issue 131
(http://www.w3.org/html/wg/wiki/ChangeProposals/CaretSelection#References)
to reference this revised change to the canvas 2D API (attached); to
include the 508 considerations for the Issue 131 change; and to reference
defect 11239.

(See attached file: HTML_Canvas_2DContext20110415.html)

Rich Schwerdtfeger
CTO Accessibility Software Group