fix(dds): Fix broken reading of volumetric DDS files by emulating tiles

Fixes 5132.

Reading DDS volumetric files used to work, but was broken by PR 4669
last year, the one where we added all the span-oriented API calls.

At that time, I did not realize/remember that DDS files could be
volumetric, observed that all the (other) volumetric files only
supported tiled access, and so in the course of that API refactor, I
removed the z parameter from the new read_scanlines API.

I don't want to break ABI compatibility again for this, so the
solution I came up with in this PR is just to make DDS volume files
report themselves as "tiled", with each tile being (width x 1 x 1),
i.e. what used to be one scanline, and it all just works out.

Signed-off-by: Larry Gritz <lg@larrygritz.com>

Author: lgritz

src/dds.imageio/ddsinput.cpp

@@ -862,6 +862,11 @@ DDSInput::seek_subimage(int subimage, int miplevel)
     // detect texture type
     if (m_dds.caps.flags2 & DDS_CAPS2_VOLUME) {
         m_spec.attribute("textureformat", "Volume Texture");
+        // Because we don't support reading scanlines from volume textures,
+        // pretend it's tiled with width x 1 x 1 tiles.
+        m_spec.tile_width = m_spec.width;
+        m_spec.tile_height = 1;
+        m_spec.tile_depth = 1;
     } else if (m_dds.caps.flags2 & DDS_CAPS2_CUBEMAP) {
         m_spec.attribute("textureformat", "CubeFace Environment");
         // check available cube map sides
@@ -1042,12 +1047,29 @@ DDSInput::read_native_tile(int subimage, int miplevel, int x, int y, int z,
     if (!seek_subimage(subimage, miplevel))
         return false;
 
+    // don't proceed if not tiled
+    if (m_spec.tile_width == 0)
+        return false;
+
+    if (m_dds.caps.flags2 & DDS_CAPS2_VOLUME) {
+        // This is a 3D volume file. For DDS files, we are emulating tiles,
+        // with each scanline as a tile.
+        if (m_buf.empty())
+            readimg_scanlines();
+        // We reported scanlines as tiles, so x must be 0.
+        if (x != 0)
+            return false;
+        size_t size   = spec().scanline_bytes();
+        size_t offset = size * (size_t(z) * size_t(m_spec.height) + y);
+        if (offset + size > m_buf.size())  // Bounds check
+            return false;
+        memcpy(data, m_buf.data() + offset, size);
+        return true;
+    }
+
     // static ints to keep track of the current cube face and re-seek and
     // re-read face
     static int lastx = -1, lasty = -1, lastz = -1;
-    // don't proceed if not a cube map - use scanlines then instead
-    if (!(m_dds.caps.flags2 & DDS_CAPS2_CUBEMAP))
-        return false;
     // make sure we get the right dimensions
     if (x % m_spec.tile_width || y % m_spec.tile_height
         || z % m_spec.tile_width)