size_t is unsigned and hence can't be below zero, triggering this gcc
warning with gcc 10:
warning: comparison of unsigned expression in ‘< 0’ is always false [-Wtype-limits]
It seems this if statement is meant to check if there are messages to
process (larger than 0). If there are no messages, we should jump out
early.
Add static for all functions only used inside the individual compile
units. This helps the compiler to potentially inline these functions.
This allows to use neatvnc as a Meson subproject in Weston which has
-Wmissing-prototypes enabled by default.