Re-land fixes Win7 configs and placement of the SwiftShader module for
ASAN/TSAN configs.

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.

Bug: angleproject:3876
Bug: b/140251624
Change-Id: I5e16057ac4de07bbdbbd248542b1b9103133294f
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1810065
Commit-Queue: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>