silence warning 'possible loss of data' (win64)

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

lodepng.cpp

@@ -3460,7 +3460,7 @@ unsigned lodepng_convert(unsigned char* out, const unsigned char* in,
                          const LodePNGColorMode* mode_out, const LodePNGColorMode* mode_in,
                          unsigned w, unsigned h)
 {
-  size_t i;
+  unsigned i;
   ColorTree tree;
   size_t numpixels = w * h;
 
@@ -4299,7 +4299,7 @@ static unsigned readChunk_bKGD(LodePNGInfo* info, const unsigned char* data, siz
 }
 
 /*text chunk (tEXt)*/
-static unsigned readChunk_tEXt(LodePNGInfo* info, const unsigned char* data, size_t chunkLength)
+static unsigned readChunk_tEXt(LodePNGInfo* info, const unsigned char* data, unsigned chunkLength)
 {
   unsigned error = 0;
   char *key = 0, *str = 0;
@@ -4343,7 +4343,7 @@ static unsigned readChunk_tEXt(LodePNGInfo* info, const unsigned char* data, siz
 
 /*compressed text chunk (zTXt)*/
 static unsigned readChunk_zTXt(LodePNGInfo* info, const LodePNGDecompressSettings* zlibsettings,
-                               const unsigned char* data, size_t chunkLength)
+                               const unsigned char* data, unsigned chunkLength)
 {
   unsigned error = 0;
   unsigned i;
@@ -4392,7 +4392,7 @@ static unsigned readChunk_zTXt(LodePNGInfo* info, const LodePNGDecompressSetting
 
 /*international text chunk (iTXt)*/
 static unsigned readChunk_iTXt(LodePNGInfo* info, const LodePNGDecompressSettings* zlibsettings,
-                               const unsigned char* data, size_t chunkLength)
+                               const unsigned char* data, unsigned chunkLength)
 {
   unsigned error = 0;
   unsigned i;