getPixelsRef -> getPixels

getPixelsRef() is a confusing method name and most beginners keep
using getPixels missing the additional functionality in ofPixels.

this deprecates all getPixelsRef methods and converts all:

```cpp
unsigned char * getPixels();
```

into

```cpp
ofPixels & getPixels();
const ofPixels & getPixels();
```

we might even want to remove the non-const version since it doesn't
really make sense to modify the internal pixels.

also adds a cast operator to unsigned char* to ofPixels so we don't break
any code except for the implementations of ofBaseHasPixels that will need
to change getPîxelsRef -> getPixels and remove the old method returning
unsigned char*