Device::stretchRect() now supports out of bounds coordinates.
To avoid linear interpolation errors, source coordinates now
have to be in floating point rather than integer format. Most
changes in this cl are just to accommodate that int->float
change for the source rect.

Fixes all (28) failures in:
dEQP-GLES3.functional.fbo.blit.rect

Change-Id: I8fd017e60b61f2d7d6517b0e648b324be441cddd
Reviewed-on: https://swiftshader-review.googlesource.com/14648Tested-by: Alexis Hétu <sugoi@google.com>
Reviewed-by: Nicolas Capens <nicolascapens@google.com>