Add upektc_img driver

Imaging driver, handles UPEK 147e:2020 device
upektc_img: add support for Upek Eikon 2 devices
upeke2: disable by default, device is now handled by upektc_img driver
imgdev: perform 5 scans for enrollment

This feature dramatically improves matching rate on devices with small
sensors.
aes1660: remove unneeded lowering of bz3_threshold

Since 5 scans for enroll was introduced it's not necessary to lower
bz3_threshold anymore, there's a good probability that scan to verify matches
with at least one enrolled sample.
vfs301: drop nr_enroll_stages override

Imaging drivers aren't allowed to change number of enroll stages anymore,
all that imaging driver does is providing image to the upper layer.
Upper layer decides if it needs more scans.
vfs101: drop nr_enroll_stages override

Imaging drivers aren't allowed to change number of enroll stages anymore,
all that imaging driver does is providing image to the upper layer.
Upper layer decides if it needs more scans.
Add EgisTec ES603 driver

This driver handles EgisTec ES603 device, ID 1c7a:0603
lib: use pixman for imaging utils

pixman is very lightweight library for pixel manipulation, and it
has no dependencies except glibc, so using it instead of gdkpixbuf/imagemagick
makes list for libfprint dependencies a bit shorter.
build: add subdir-objects option to make automake happy

Without subdir-object automake throws warnings like this one:

Makefile.am:x: warning: source file 'drivers/aes2550.c' is in a subdirectory,
Makefile.am:x: but option 'subdir-objects' is disabled