A call to glDrawElements results in a calling depth of up to 4
* glDrawElements
* gl::Context::DrawElements
* rx::ContextGL::DrawElements
* VertexArrayGL::syncDrawState.

Each function call has to save/restore a lot of registers which
results in a stall in the prologue of rx::ContextGL::DrawElements
due to memory bandwidth limitations.

The main change is the function gl::Context::DrawElements being
inlined to reduce the calling depth by one. In addition the call
to ContextGL::syncDrawElementsState is now protected so that it
gets called only if it's required. Finally a few small getter
functions have been inlined where the calling code was bigger
than the actual function.

In total this change improves performance of the
DrawElementsPerfBenchmark.Run/gl benchmark by 16%.

Bug: angleproject:2966

Change-Id: I423d18452f2f5b520ab52850fda2054e1da86991
Reviewed-on: https://chromium-review.googlesource.com/c/1389988Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Markus Tavenrath <matavenrath@nvidia.com>