Removed early return on source rectangle validation, in order
to allow out of bounds coordinates as an input to the blitter.

Fixes 31 *out_of_bounds* failures in:
dEQP-GLES3.functional.fbo.blit.default_framebuffer

Also fixes 3 scissored blit failures in:
dEQP-GLES3.functional.fragment_ops.scissor

Change-Id: I0751678153aa0fc58bb7aa3a0270c358efb61330
Reviewed-on: https://swiftshader-review.googlesource.com/15388Tested-by: Alexis Hétu <sugoi@google.com>
Reviewed-by: Nicolas Capens <nicolascapens@google.com>