Do not copy pixels in a negative read region. Also make sure we clamp
the boundaries so the dEQP tests are mostly pixel-perfect.

We can't yet fix some default framebuffer tests because of issues with
blitting RGBA8 (default FBO) to floating point framebuffers.

BUG=angleproject:1241
TEST=dEQP-GLES3.functional.fbo.blit.*

Change-Id: I16ee264b0b1a7e6b9121dde5ae4f96cfd27ea53e
Reviewed-on: https://chromium-review.googlesource.com/315670Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Tested-by: Jamie Madill <jmadill@chromium.org>