fix out-of-bounds write in store_dds_uncompressed_image for 3d ldr

[sahvx655-wq]

While checking the uncompressed DDS writer against the KTX one I noticed the LDR plane pointer in store_dds_uncompressed_image is advanced by dim_y * dim_z * image_components per slice, where a plane is actually dim_x * dim_y * image_components. The 16-bit branch in the same function, and both branches of the KTX writer, use dim_x, so this is a stray dim_z. For any 3D image whose depth is larger than its width the per-slice base pointer lands past the end of the pixel_data8 buffer and the scanline copy then writes over the heap. It is reachable when decoding a 3D LDR .astc straight to .dds, or when saving a 3D image array as .dds.

AddressSanitizer reports a heap write one byte past the pixel buffer on a 4x4x8 LDR image, traced back to this slice loop; with the stray dim_z swapped back to dim_x every plane pointer sits inside the buffer and the writer round-trips cleanly. The bound belongs in the writer because the buffer being indexed is the one this function allocates from the same dimensions, so a caller has no way to correct a stride computed internally.