the inputlayout incorrectly described the POSITION attribute as being
4D, while ImDrawVert::pos is 2D. This went unnoticed because the buffer
binding has a stride of sizeof(ImDrawVert) and the POSITION is treated
as a float2 in the vertex shader. If you switch POSITION to float4 in
the vertex shader (and actually use the z/w in the matrix
multiplication) then everything become wacky-looking since it's
interpreting the texture coordinates as z/w. On a similar note: It's
weird that the projection matrix takes z and w into consideration when
those don't exist in the shader due to positions being float2s.