Adds a new EGL extension for picking SwiftShader when using the Vulkan
back-end. Also cleans up ICD enabling code RendererVk. Also includes a
change to a buffer size necessary to support SwiftShader's minimum
limits.

32-bit is currently left disabled to work around an issue on AMD Win7.

Bug: angleproject:3876
Bug: b/140251624
Change-Id: I33c55d994801d6154caca0cad0a608c1e808d517
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1776231
Commit-Queue: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>