Essentially, do not assume CMAKE_SOURCE_DIR is the SwiftShader root
directory. Instead, we now set SWIFTSHADER_DIR in the root CMakeLists,
and use that everywhere.

Bug: b/145758253
Change-Id: I4968d0dc06ebd8e45791f7cd956e5aeeefddd54f
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/43949
Kokoro-Result: kokoro <noreply+kokoro@google.com>
Reviewed-by: Ben Clayton <bclayton@google.com>
Reviewed-by: Nicolas Capens <nicolascapens@google.com>
Tested-by: Antonio Maiorano <amaiorano@google.com>