avutil/log: merge calls that set ansi color and print the string

about 1/3 or so faster

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
avutil/log: skip setting color for default (AV_LOG_INFO) level

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
avutil/log: skip IO calls on empty strings

These occur when no context is set for example, thus they are common

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>