1. Compare updating values with active ones at frontend when
depth&stencil related gl call happens and do not set any unnecessary
dirty bits.
2. Remove d3d and gl backends' checking of depth&stencil states when
detect any dirty bits, as it is now being done at frontend.
3. Modification to graphics pipeline description update, make sure it
syncs with gl::State initial value.
4. Change gl_vk::CullMode return type to VkCullModeFlagBits, as it only
represents single value.

Bug: angleproject:3700
Change-Id: Id3aa5186455ee3a10a9c147edad13944e3e41098
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1706903
Commit-Queue: Jiacheng Lu <lujc@google.com>
Reviewed-by: Tobin Ehlis <tobine@google.com>