Fix segfault due to invalid masks generation. Fixes nmoehrle#125.

Until now, it was possible that a mask with a 255 in its border was generated,
later failing the `valid_mask` assertions, or, if assertions are disabled
by the build, segementation faults due to invalid memory accesses.

See the example in the added comment for a condition where this could happen.

The key insight is that if a point lay exactly on the "border" between two
pixels, say between pixel N and N+1, it counted as occupying pixel N+1.

So the triangle { (1,1), (1,2), (2,1) } in a 3x3 image would result in mask

    64   64   64
    64  255  255
    64  255   64

(notice the triangle of 255s), instead of the correct mask

    64   64   64
    64  255   64
    64   64   64

This commit fixes it by adding the condition that the last
`x = width-1` and `y = height-1` must not count as `inside` the triangle.

It also improves related assertions in a few places.

Author: nh2

libs/tex/poisson_blending.cpp

@@ -20,7 +20,10 @@ typedef Eigen::SparseMatrix<float> SpMat;
 
 math::Vec3f simple_laplacian(int i, mve::FloatImage::ConstPtr img){
     const int width = img->width();
-    assert(i > width + 1 && i < img->get_pixel_amount() - width -1);
+    // Check it's not in the top/bottom border (`texture_patch_border = 1`).
+    assert(i > width && i < img->get_pixel_amount() - width);
+    // Check it's not in the left/right border (`texture_patch_border = 1`).
+    assert(i % width != 0 && i % width != width-1);
 
     return -4.0f * math::Vec3f(&img->at(i, 0))
         + math::Vec3f(&img->at(i - width, 0))

libs/tex/texture_patch.cpp

@@ -67,9 +67,28 @@ TexturePatch::adjust_colors(std::vector<math::Vec3f> const & adjust_values) {
             for (int x = min_x; x < max_x; ++x) {
 
                 math::Vec3f bcoords = tri.get_barycentric_coords(x, y);
-                bool inside = bcoords.minimum() >= 0.0f;
+                // The point (x, y) is inside the triangle if all of its
+                // barycentric coordinates are positive.
+                // Further, we need to check that (x, y) does not touch the
+                // upper `texture_patch_border` (width-1 and height-1).
+                // This isn't necessary for the lower boarders.
+                // Example:
+                // Consider the triangle { (0,0), (0,1), (1,0) }
+                // in a 1x1 pixel image. With 1-pixels borders added around it
+                // it becomes the triangle { (1,1), (1,2), (2,1) }
+                // in a 3x3 pixel image.
+                // The lower borders (rows x=0 and y=0) will be free as expected
+                // because no point has these coordinates.
+                // But the upper border x=2 is "just touched" by point (2,1),
+                // so it would be marked as occupied (and texture borders are
+                // intended to be not occupied).
+                // Thus we must exclude them explicitly in the `inside`
+                // condition.
+                bool inside = bcoords.minimum() >= 0.0f
+                    && x != get_width() - 1 && y != get_height() - 1;
                 if (inside) {
                     assert(x != 0 && y != 0);
+                    assert(x != get_width() - 1 && y != get_height() - 1);
                     for (int c = 0; c < 3; ++c) {
                         iadjust_values->at(x, y, c) = math::interpolate(
                             adjust_values[i][c], adjust_values[i + 1][c], adjust_values[i + 2][c],