Prevent blur artefacts when surface is cropped

Bug: 417192419
Test: Manual, i.e. verified that blur works as expected during Floaty
      invocation without any artefacts (except at the corners)
Flag: com.android.graphics.surfaceflinger.flags.window_blur_kawase2_fix_aliasing
Change-Id: I5c126bc0c18935db95eeec1f75bf2f99dd0933a0