add stereo source support in EqualPowerPanner

add stereo source support in EqualPowerPanner
https://bugs.webkit.org/show_bug.cgi?id=81092

Patch by Wei James <james.wei@intel.com> on 2012-04-11
Reviewed by Chris Rogers.

Source/WebCore:

Test: webaudio/panner-equalpower-stereo.html

* platform/audio/EqualPowerPanner.cpp:
(WebCore::EqualPowerPanner::pan):

LayoutTests:

* webaudio/panner-equalpower-stereo-expected.txt: Added.
* webaudio/panner-equalpower-stereo.html: Copied from LayoutTests/webaudio/panner-equalpower.html.
* webaudio/panner-equalpower.html:
* webaudio/resources/audio-testing.js:
(createStereoImpulseBuffer):
* webaudio/resources/panner-model-testing.js:
(createGraph):
(createTestAndRun):
(equalPowerGain):
(checkResult):

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@113889 268f45cc-cd09-0410-ab3c-d52691b4dbfc