Prevent leaking memory on cursors

[LeonSkoog]

Hello,
It's my first PR to this project, I don't know what is the correct procedure yet :)

During the Init process, bunch of cursors being allocated:

// load mouse cursors
loadMouseCursor(ImGuiMouseCursor_Arrow, sf::Cursor::Arrow);
loadMouseCursor(ImGuiMouseCursor_TextInput, sf::Cursor::Text);
loadMouseCursor(ImGuiMouseCursor_ResizeAll, sf::Cursor::SizeAll);
loadMouseCursor(ImGuiMouseCursor_ResizeNS, sf::Cursor::SizeVertical);
loadMouseCursor(ImGuiMouseCursor_ResizeEW, sf::Cursor::SizeHorizontal);
loadMouseCursor(ImGuiMouseCursor_ResizeNESW, sf::Cursor::SizeBottomLeftTopRight);
loadMouseCursor(ImGuiMouseCursor_ResizeNWSE, sf::Cursor::SizeTopLeftBottomRight);
loadMouseCursor(ImGuiMouseCursor_Hand, sf::Cursor::Hand);

loadMouseCursor function allocates memory for cursor and marks whether loading the cursor succeed or not:

void loadMouseCursor(ImGuiMouseCursor imguiCursorType, sf::Cursor::Type sfmlCursorType) {
     s_currWindowCtx->mouseCursors[imguiCursorType] = new sf::Cursor();
     s_currWindowCtx->mouseCursorLoaded[imguiCursorType] =
     s_currWindowCtx->mouseCursors[imguiCursorType]->loadFromSystem(sfmlCursorType);
}

Destructor deletes only cursors marked as 'loaded':

     ~WindowContext() {
         delete fontTexture;
         for (int i = 0; i < ImGuiMouseCursor_COUNT; ++i) {
             if (mouseCursorLoaded[i]) {
                 delete mouseCursors[i];
             }
         }
         ImGui::DestroyContext(imContext);
    }

However, it's very much possible that the cursor couldn't be loaded and the space for it won't be de-allocated. Which leaks the memory.

My PR is a proposal of handling the memory leak.

[lefticus]

I just hit this exact issue while trying to demo some sfml/imgui work with ASAN enabled.

[eliasdaler]

That's a good catch!
Sorry it took me so long to take to this. Thanks for the PR!