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
fix(ui): fix dynamic menu height calculation
fix(app): initilize menu properly in reader and filemanager
fix(reader): crash on closing reader menu
Merge pull request #1041 from houqp/dxg

more fixes for none touch kindles
test: add spec for Menu.itemTableFromTouchMenu
build: supress make clean output
chore: readme clean up
build: update koreader base for legacy kindle build fix
chore: code clean up
Leave both readerconfig and readermenu open on wifi toggle
Merge pull request #1047 from houqp/kindle-legacy

Kindle legacy
Merge pull request #1049 from gontofe/wifi_toggle

Leave both readerconfig and readermenu open on wifi toggle
Added option for proportional cover image screensaver
Merge pull request #1051 from gontofe/cover_screensaver

Added option for proportional cover image screensaver