Adds a Framebuffer class to encapsulate and abstract framebuffer
creation with or without multisampling.

Bug: b/158231104
Change-Id: I4198f53f872aae76955ba13f1f2372067b91a07c
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/45789
Presubmit-Ready: Nicolas Capens <nicolascapens@google.com>
Tested-by: Nicolas Capens <nicolascapens@google.com>
Kokoro-Result: kokoro <noreply+kokoro@google.com>
Reviewed-by: Antonio Maiorano <amaiorano@google.com>