silence warning 'possible loss of data' (win64)

[haubi]

When compiling for 64bit Windows, size_t and unsigned int are of different size, leading cl.exe to warning C4267: conversion from 'size_t' to 'unsigned int', possible loss of data #34 contains a different solution, but this one feels more straightforward.

[haubi]

Comparing with size_t doesn't generate a warning (no data loss actually), and the patch changes the readChunk_??Xt() functions passing i as argument to to unsigned as well.

The warnings start with passing i to color_tree_add(), which casts the unsigned index to signed. And the readChunk_??Xt() take size_t chunkLength , but assign to unsigned length as well, triggering the warning.

Alternative would be to use ssize_t (signed size_t) for the ColorTree::index, but ssize_t is not available on some platforms, nor is it used anywhere within lodepng yet.

[stinos]

Good point, should have checked more in detail. I think it would be better to fix all warnings at once though: when compiled with /W4 there's still a bucnh of conversion issues left.