Basic shell class for Framebuffer

Bug b/119621736

Change-Id: Iaf5466d311efe011ea7bbd4a6e3ab2802474f98e
Reviewed-on: https://swiftshader-review.googlesource.com/c/22611Tested-by: Alexis Hétu <sugoi@google.com>
Reviewed-by: Nicolas Capens <nicolascapens@google.com>