wined3d: Do not take "draw_rect" into account when comparing attachment sizes in ffp_blitter_clear().

The scissor rectangle and viewport shouldn't affect whether attachment sizes
are equal. The Intel i965 driver will do a fast clear when the scissor
rectangle overlaps the intersection of the attachments, and that behaviour is
allowed by the OpenGL spec. This commit fixes failures in the d3d8 and d3d9
depth_buffer_test() tests on the Intel i965 driver.

See also commit 0530f33c.
Signed-off-by: Henri Verbeet <hverbeet@codeweavers.com>
Signed-off-by: Alexandre Julliard <julliard@winehq.org>