Fix clearing of dirty textures.

Fix clearing of dirty textures.

When clear operations fall back to the slow path (i.e. neither fastClear
nor blitReactor is used), we were copying a rectangle the size of the
destination image. It should only sample within the 1x1 source pixel
instead.

Bug chromium:852641, chromium:851707

Change-Id: I9f247483f6167f92be8308b8470c021f5641b657
Reviewed-on: https://swiftshader-review.googlesource.com/19448
Reviewed-by: Alexis Hétu <sugoi@google.com>
Tested-by: Nicolas Capens <nicolascapens@google.com>