This tutorial implements very basic shadow mapping: the scene is rendered as
seen from the light source, the depth buffer of the result is stored in a
texture. Then the scene is rendered as seen from the camera, and in the vertex
shader we calculate both the light and camera projection of the vertex. In the
fragment shader, we can look up the depth value from the shadow map texture,
and if it is lower (ie, closer to the light source) than the current fragment
is as seen from the light source, it must be in shadow.

Several parameters can be changed while running the program, to show trade-offs
between the light source FOV, shadow map size and whether to cull front or back
faces during the calculation of the shadow map.