use more precise RGB to gray conversion

we use this:
gray8 = rshift(4897*red + 9617*green + 1868*blue, 14)
Merge pull request #93 from hwhw/blitbuffer-conversion

use more precise RGB to gray conversion
add back missed sample jpg file for unit test
fix: convert emulator dimension from string to number

* also add comment in blitbuffer module
Merge pull request #94 from houqp/houqp-master

convert emulator dimension from string to number
Merge remote-tracking branch 'upstream/master' into ffi-framebuffer-native