Default canvas backing store to be 1:1 with specified dimensions.

Default canvas backing store to be 1:1 with specified dimensions.
https://bugs.webkit.org/show_bug.cgi?id=78971

Reviewed by Adam Barth.

Although the canvas specification states that the backing image may
be larger than the user-specified dimensions, there are a number of
philip canvas tests that fail when the backing image data is not
1:1 with the specified canvas dimensions. These failures are
tracked in https://bugs.webkit.org/show_bug.cgi?id=73645

This change defaults the canvas backing store to be 1:1 with the
user-specified dimensions, while also providing an
ENABLE(HIGH_DPI_CANVAS) build option to reinstate the original
behaviour, so that the above bug may be more easily fixed.

* html/HTMLCanvasElement.cpp:
(WebCore::HTMLCanvasElement::HTMLCanvasElement):

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@108293 268f45cc-cd09-0410-ab3c-d52691b4dbfc