ImGUI extension should require the user set Viewport and Scissor state outside of `render()` method

[devshgraphicsprogramming]

We have this piece of code which is not elegant / resillient

Nabla/src/nbl/ext/ImGui/ImGui.cpp

Line 1459 in bed64ec

SViewport const viewport

However for the user to be able to set this sanely, one must know the framebuffer Width and Height

Nabla/src/nbl/ext/ImGui/ImGui.cpp

Line 1167 in bed64ec

float const frameBufferWidth = drawData->DisplaySize.x * drawData->FramebufferScale.x;

and this seems only knowable from ImGUI::GetDrawData

Nabla/src/nbl/ext/ImGui/ImGui.cpp

Line 1161 in bed64ec

auto const* drawData = ImGui::GetDrawData();

Which probably needs a call to ImGUI::Render 🤦

P.S. also while you're at it, make the push constant setting an overridable lambda (so people can pump the data to the drawcall differently)

Nabla/src/nbl/ext/ImGui/ImGui.cpp

Line 1496 in bed64ec

commandBuffer->pushConstants(m_pipeline->getLayout(), IShader::E_SHADER_STAGE::ESS_VERTEX | IShader::E_SHADER_STAGE::ESS_FRAGMENT, 0u, sizeof(constants), &constants);

std::function<void(IGPUCommandBuffer*,const PushConstants&)>