Revert "lanczos3: Actually sample 7x7 instead of 6x6 (#27)"

This reverts commit 22e1bb1.

The kernel size of the `lanczos3` filter is 6x6, and sampling it at
`x=3.5` or `y=3.5` results in a weight of `0`, thus making these pixels
completely irrelevant.  This became more clear in #28 that simplified
the offset passed to `lanczos3_filter()` to always be `0.5`, to read the
weight at the middle of each source pixel.

Note that for an even reduction in image size the center coordinate
of every target pixel (what `uv` denotes) is exactly on the boundary
between two source pixels, meaning the pixel at kernel position
`x=0,y=0` (barring float imprecisions) is at the right/bottom of the
center of the target pixel, hence correctly reading 3 pixels to the
left, top, right and bottom (with indices in the range [-3, 2]).

For uneven reductions (i.e. 3x) this doesn't hold, and that was likely
what the code removed in #28 was incorrectly trying to compensate for?

src/ispc/kernels/lanczos3.ispc

@@ -46,12 +46,15 @@ static inline uint8<4> resample_internal(uniform Image src_image, float<2> uv, u
     // Truncate floating point coordinate to integer:
     const int<2> src_coord = uv * src_image.size;
 
-    for (uniform int x = -3; x <= 3; x++) {
-        for (uniform int y = -3; y <= 3; y++) {
-            const float lanczos3_offset = 0.5;
+    // Sample the lanczos3 filter from -2.5 to 2.5.
+    // Pixel 0, 0 in the source will be at the bottom-right of the center of the target pixel,
+    // hence reading source pixels in the range [-3, 2] will correctly read pixels to the left, top,
+    // right and bottom of the target pixel.
+    for (uniform int x = -3; x < 3; x++) {
+        for (uniform int y = -3; y < 3; y++) {
+            float wx = lanczos3_filter((uniform float)x + 0.5);
+            float wy = lanczos3_filter((uniform float)y + 0.5);
 
-            float wx = lanczos3_filter((uniform float)x + lanczos3_offset);
-            float wy = lanczos3_filter((uniform float)y + lanczos3_offset);
             float w = wx * wy;
             int<2> texel_offset = {x, y};
             int<2> src_kernel_coord = src_coord + texel_offset;