Update cimgui to 1.53.1

Update cimgui, fixing imgui-rs#97 and imgui-rs#88.

As a side note, the previous version has some compiler warning showing
up during build:

warning: third-party/cimgui/imgui/imgui.cpp: In function ‘void ImGui::RenderTriangle(ImVec2, ImGuiDir, float)’:
warning: third-party/cimgui/imgui/imgui.cpp:3136:11: warning: this statement may fall through [-Wimplicit-fallthrough=]
warning:          r = -r; // ...fall through, no break!
warning:          ~~^~~~

The new version has no warning.