PAD repeat support for fast scaling with nearest filter

PAD repeat support for fast scaling with nearest filter

When processing pixels from the left and right padding, the same
scanline function is used with 'unit_x' set to 0.

Actually appears that gcc can handle this quite efficiently. When
using 'restrict' keyword, it is able to optimize the whole operation
performed on left or right padding pixels to a small unrolled loop
(the code is reduced to a simple fill implementation):

    9b30:       89 08                   mov    %ecx,(%rax)
    9b32:       89 48 04                mov    %ecx,0x4(%rax)
    9b35:       48 83 c0 08             add    $0x8,%rax
    9b39:       49 39 c0                cmp    %rax,%r8
    9b3c:       75 f2                   jne    9b30

Without 'restrict' keyword, there is one instruction more: reloading
source pixel data from memory in the beginning of each iteration. That
is slower, but also acceptable.