ARM: NEON: source image pixel fetcher can be overrided now

ARM: NEON: source image pixel fetcher can be overrided now

Added a special macro 'pixld_src' which is now responsible for fetching
pixels from the source image. Right now it just passes all its arguments
directly to 'pixld' macro, but it can be used in the future to provide
a special pixel fetcher for implementing nearest scaling.

The 'pixld_src' has a lot of arguments which define its behavior. But
for each particular fast path implementation, we already know NEON
registers allocation and how many pixels are processed in a single block.
That's why a higher level macro 'fetch_src_pixblock' is also introduced
(it's easier to use because it has no arguments) and used everywhere
in 'pixman-arm-neon-asm.S' instead of VLD instructions.

This patch does not introduce any functional changes and the resulting code
in the compiled object file is exactly the same.