Currently, all our host unit-tests that have rendering concerns use the software backend because of OpenGL ES availability and stability issues on the various platforms where we run host tests. Unfortunately, entire subsystems are disabled (and not tested) when rendering with the software backend. This patch pulls in SwiftShader and via pending patches in the buildroot, configures the host unit-tests to optionally use OpenGL ES in a stable manner without relying on the OpenGL drivers being present (and functional).
I have wired up the embedder test fixture in this patch to use the SwiftShader based OpenGL ES driver. I will update the shell and runtime unittests in a subsequent patch as well. The on and offscreen surfaces are configured as 1x1 pbuffer surface because we should be able to write pixel tests using OpenGL directly wihout having to deal with surfaces.
The GetMapping calls removed in this patch had the same code and had to be repeated across different test harnesses as well as in dart_snapshot.cc. Just make this a factory method so the code is less verbose.
Some components in the Flutter engine were derived from the forked blink codebase. While the forked components have either been removed or rewritten, the use of the blink namespace has mostly (and inconsistently) remained. This renames the blink namesapce to flutter for consistency. There are no functional changes in this patch.
