Reformat a bit, add MSVS checks, add SSE4

intel/intel_init.c

@@ -16,6 +16,13 @@
 void
 png_init_filter_functions_sse2(png_structp pp, unsigned int bpp)
 {
+   // The techniques used to implement each of these filters in SSE operate on
+   // one pixel at a time.
+   // So they generally speed up 3bpp images about 3x, 4bpp images about 4x.
+   // They can scale up to 6 and 8 bpp images and down to 2 bpp images, 
+   // but they'd not likely have any benefit for 1bpp images.
+   // Most of these can be implemented using only MMX and 64-bit registers,
+   // but they end up a bit slower than using the equally-ubiquitous SSE2.
    if (bpp == 3)
    {
       pp->read_filter[PNG_FILTER_VALUE_SUB-1] = png_read_filter_row_sub3_sse2;