[devel] Add tests for expand16, more fixes for palette image tests

 Changes only pngvalid and test-pngvalid-full.sh - no libpng changes -
 corrects the code for palette image tests, disables attempts to validate
 palette colors.

test-pngvalid-full.sh

@@ -7,18 +7,9 @@ echo >> pngtest-log.txt
 echo "============ pngvalid-full.sh ==============" >> pngtest-log.txt
 
 echo "Running test-pngvalid-full.sh"
-for gamma in threshold transform sbit 16-to-8 background alpha-mode
+for gamma in threshold transform sbit 16-to-8 background alpha-mode "transform --expand16" "background --expand16" "alpha-mode --expand16"
 do
-   # For the moment the composition calculation is performed with minimal
-   # accuracy, do this to work round the problem:
-   if test $gamma = background -o $gamma = alpha-mode
-   then
-      opts=--use-linear-precision
-   else
-      opts=
-   fi
-
-   if ./pngvalid $opts "--gamma-$gamma" >> pngtest-log.txt 2>&1
+   if ./pngvalid --gamma-$gamma >> pngtest-log.txt 2>&1
    then
       echo "  PASS:" pngvalid "--gamma-$gamma"
    else