protected call of mupdf.renderImage
fix #1028
update base to use new libcrengine
Merge pull request #1030 from chrox/fix_1028

fix #1028
install nasm in Travis
Merge pull request #1031 from chrox/travis_nasm

install nasm in Travis
fix #1028 by trying load libpthread.so.0 on Kobo devices
Merge pull request #1032 from chrox/fix_1028

fix #1028 by trying load libpthread.so.0 on Kobo devices
unify color specification

colors were a mixture of 4bpp integers (0=white, 15=black) and
fractional blackness levels (0=white, 1.0=black) before. This is
now unified to use the color specification of the Blitbuffer API.
fill background of intermediary blitbuffers

we need to explicitly fill those buffers, now that white is not "0"
anymore.
update to latest base
Merge pull request #1036 from hwhw/noninvert

Unroll inversion, introduce color values