v1: Heatmap of f(x,y) with colormap in compute code

Computes the heatmap plot of a function $z=f(x,y)$ of two parameters,
generating RGBA pixmap, which is then used as a texture to draw the
result.  The compute kernel (be it on CPU or on GPU) is aware and knows
of the RGBA 8-bits per channel pixel format (unsigned char per channel).

The function to draw is hardcoded in plot2d_common.hpp as `func(x,y)`,
and cannot be changed at runtime.  Similarly, colormap is hardcoded
inside `plotPixel()` function, which also implements min-max rescaling.

Simple grayscale colormap (R = G = B = 255*rescaled value, A = 255) with
overflow marking (blue for values smaller than expected minimum value,
red for values above expected maximum) is used.

This code borrows from 'func2d.cu', 'gpu_bitmap_static.cu' and
'gpu_bitmap_anim.cu' from the func2d-glut [1][] project.

[1]: https://gitlab.com/programowanie-gpu/func2d-glut