[C++] Remove const qualifier from Subscription::imageAtIndex.
[C++] Remove const qualifier from Subscription::imageAtIndex.

mikeb01/Aeron,real-logic/Aeron,EvilMcJerkface/Aeron,galderz/Aeron,mikeb01/Aeron,EvilMcJerkface/Aeron,galderz/Aeron,mikeb01/Aeron,real-logic/Aeron,real-logic/Aeron,galderz/Aeron,mikeb01/Aeron,galderz/Aeron,EvilMcJerkface/Aeron,real-logic/Aeron,EvilMcJerkface/Aeron