Makefile: only build libpldraw.a, not libpldraw.so

Now that all the PLSDK libraries are packaged into a single
libplsdk.so, there is no need to keep individual shared
libraries.
improve get_color implementation

Instead of testing the framebuffer pixel format and CFA
configuration for each call to get_color, make separate functions
with a pointer in the pldraw instance.  The test is only made
once when the instance is initialised and when the CFA setting is
changed.

This improves code maintainability and performance when calling
get_color in a loop.

+ add _PLDRAW_PIX_N_ to know the number of colour mode
  identifiers and use it when initialising tables
add setver.sh script to set the library version string

This script takes care of updating the library version
string in libpldraw.h and adds it to the git index for
automatic version update when running release script.
ChangeLog v1.7 -> v1.8