Fix memory/resource handling in imgdiff.cpp, using unique_ptr and vector.

This fixes the following cppcheck reports:
[imgdiff.cpp:197]: (error) Memory leak: img
[imgdiff.cpp:386]: (error) Memory leak: img
[imgdiff.cpp:656]: (error) Memory leak: data
[imgdiff.cpp:664]: (error) Memory leak: data
[imgdiff.cpp:668]: (error) Memory leak: data
[imgdiff.cpp:668]: (error) Resource leak: f
[imgdiff.cpp:820]: (error) Memory leak: bonus_data
[imgdiff.cpp:824]: (error) Memory leak: bonus_data
[imgdiff.cpp:824]: (error) Resource leak: f
[imgdiff.cpp:847]: (error) Memory leak: bonus_data
[imgdiff.cpp:851]: (error) Memory leak: bonus_data
[imgdiff.cpp:856]: (error) Memory leak: bonus_data
[imgdiff.cpp:860]: (error) Memory leak: bonus_data
[imgdiff.cpp:227]: (error) Memory leak: temp_entries

Change-Id: I06f878d0b677a25328e0deb84f65f3b7212e24b1