Fix implementation format for default renderbuffer.

Fix implementation format for default renderbuffer.

The spec defines that the initial state for a newly bound renderbuffer
has a format of RGBA4. However, when querying the number of red/green/
blue/alpha/depth/stencil bits, we should report the actual number of
bits allocated (0), and not assert.

Bug chromium:765912
Bug b/25351344

Change-Id: Icc3eb59e76bcbd6f0f2f16a2a75ace706ae17251
Reviewed-on: https://swiftshader-review.googlesource.com/12148
Reviewed-by: Alexis Hétu <sugoi@google.com>
Reviewed-by: Nicolas Capens <nicolascapens@google.com>
Tested-by: Nicolas Capens <nicolascapens@google.com>