Fix missing argument in fill_noop in CPU backend
Reactivate bounds checks for debugging G2D functions

Add extra bounds checks in G2D fill and blit functions to investigate
potential issues. Any warning generated by these checks will cause
the X server to exit.
Allow CPU backend to be called when G2D is enabled

To optimize performance when G2D blit has to fall back to CPU
blit, allow the NEON optimized CPU back-end to be called from
sunxi_x_g2d.c. An extra pointer to the cpu_backend blt2d_i structure
is added to the SunxiG2D structure to accomplish this. In case only
the CPU back-end is used and not G2D, the extra cpu_backend pointer
is NULL. Using G2D without the CPU backend is also supported.

This dramatically improves performance when G2D is enabled for
rightwards overlapped copies (dragging windows to the right), as
can shown with benchx (https://github.com/hglm/benchx) using the
CopyScreenRightwards test.