Fixes & improvements to Image, including scaling & arbitrary rotation.

[neildavis]

This commit adds bilinear interpolation scaling and arbirary rotation methods to Image.
In the process, several issues with Image were found & fixed:

• There was no destructor to free the canvas memory. This would cause a memory leak unless the client remembered to call close() A virtual destructor has been added to free this memory.

• There was no explicit copy & move constructor for Image This means whenever Image was copied or passed by value a 'shallow copy' was performed resulting in both instances referencing the same shared canvas memory. This works, and several examples were depending on it, but after the addition of the destructor it caused seg faults since the memory was free()'d several times. An explicit copy constuctor for Image has been added to perform a 'deep copy' of the canvas memory. A C++11 'move' constructor has been added to avoid excessive deep copies being made when passing Image as a temporary.

• The same logic for copy/move constructors also applies to assignment operators (operator=()) so these are added.

• Image (and Color) are extensively passed by value within the library. This now potentially inefficient due to the deep copy. (if the move c'tor is not used) Since pass-by-reference semantics are required in these cases they are now passed by reference.

• Pass-by-(const)-reference changes also required many methods on Image and Color that did not modify their members to be declared const as well as all Color preset constants.

• New c'tors and getters added to Color to support RGBA access as required by the new scaling method on Image Also tidied up c'tors to use common helper methods.

• Used new/delete instead of malloc/free for heap memory use in Image Just coz it's more conventional in C++