This moves the skipDraw logic into applyPrimitiveType, since it's
more efficient to only check the primitive type once. Also merges
the draw*Impl and genericDraw* methods, since the generic* methods
weren't really doing anything anymore, and all the state logic lives
in StateManager::updateState.

This series of small optimizations gives about a 15% improvement on
the draw call benchmark for the D3D11 backend with the null driver.

BUG=angleproject:1155

Change-Id: I299213da6d1bbcb08691d5b50162e6cae16cb4bb
Reviewed-on: https://chromium-review.googlesource.com/666044Reviewed-by: Geoff Lang <geofflang@chromium.org>
Commit-Queue: Jamie Madill <jmadill@chromium.org>