In ES 3.0 and 3.1, only non-indexed GL_POINTS, GL_LINES and GL_TRIANGLES
is supported for transform feedback.  Without tessellation and geometry
shaders, we can calculate the exact location where each vertex transform
output should be written on the CPU, and have each vertex shader
invocation write its data separately to the appropriate location in the
buffer.

This depends on the vertexPipelineStoresAndAtomics Vulkan feature.

Bug: angleproject:3205
Change-Id: I68ccbb80aece597cf20c557a0aee842360fea593
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1645678
Commit-Queue: Shahbaz Youssefi <syoussefi@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>