SwiftShader currently always preserves the color buffer on eglSwapBuffers,
so both EGL_BUFFER_PRESERVED and EGL_BUFFER_DESTROYED can be used without
real changes.

Change-Id: I55fcd7d4b4211483d8876c15d3d6ea77ea2685e6
Reviewed-on: https://swiftshader-review.googlesource.com/2721Reviewed-by: Ping-Hao Wu <pinghao@google.com>
Reviewed-by: Nicolas Capens <capn@google.com>
Tested-by: Nicolas Capens <capn@google.com>