Avoid function-call overhead in CopyArea for some common cases
Add performance comparison with ShadowFB enabled
Optimize conditionals in X driver fill function

Probably due to branch-prediction characteristics of the ARM CPU, it is
sensitive to the way conditional statements are ordered. This commit
set the fill function to NULL if it is not implemented instead of having
a no-op functions returning 0. The fill function of the X-driver tries
to minimize the conditional penalties, resulting in improved performance
for small off-screen fills:

PixmapFillRect (5 x 5): Speed up 26%
PixmapFillRect (7 x 7): Speed up 30%
PixmapFillRect (10 x 10): Speed up 25%
PixmapFillRect (15 x 15): Speed up 8%
PixmapFillRect (22 x 22): Speed up 6%
Optimize conditional statements in CopyArea

This commit tries to optimize the conditionals in CopyArea; it doesn't
really make a difference, but it seems fix a small performance regression
introduced by a previous commit (common case optimization for CopyArea).
Add ARM-optimized 32-byte multiple memcpy routine

Add an optimized assember function used for writing to scratch memory
in screen-to-screen blits. This patch also adds -march=armv6j to
CFLAGS; not sure whether it helps.
Add standard blit to device-independent blt2d structure

Add a new function, standard_blt, to the device-independent
acceleration interface. The intention is to provide an optimized
blit function for the case that the source is not in the framebuffer
(the latter case is handled by overlapped_blt).
Disable assembler blit optimization because it is not yet functional
Add experimental standard blit routines

Blit functions are provided for the standard blit hooks. Written in C,
they haven't been debugged properly and are considerably slower than
pixman, which is enabled by default.