Added getMaxRenderTargets() to Renderer, for a runtime-specific value for the maximum simulatenous render targets.

We will report this as our maximum draw buffer and color attachment count.

TRAC #22656

Signed-off-by: Nicolas Capens
Signed-off-by: Shannon Woods
Author: Jamie Madill

git-svn-id: https://angleproject.googlecode.com/svn/branches/dx11proto@2004 736b8ea6-26fd-11df-bfd4-992fa37f6226