exynos: fix font rendering

It would just segfault after a while, since we weren't deferencing
the 'msg' pointer to check for the end of the string.
Eventually this would let 'msg' point to invalid memory.
exynos: font: use interpolate mode for blending

The interpolate mode is more suited for font rendering, even
if it produces slightly different results.
The main reason for switching is to avoid some conditionals
in the coming code improvements.
exynos: font: slight improvement to rendering performance

The current bottleneck is 'put_glyph_rgba4444'. Interpolate mode
enables us to remove the conditional in the inner loop.

Eventually this call should just reduce to a memcpy.
exynos: font: compute color in ARGB4444 format just once
Merge pull request #859 from tobiasjakobi/fixes

exynos: first batch of fixes/improvements for font rendering