Stencil formats were prohibited from createDepthStencilSurface
in debug builds due to an assert, so fixed the assert.

Change-Id: I5777a081371be4f43810de4450b415c55f04a8d7
Reviewed-on: https://swiftshader-review.googlesource.com/16348Tested-by: Alexis Hétu <sugoi@google.com>
Reviewed-by: Nicolas Capens <nicolascapens@google.com>