Autodetect screen resolution

Always run at full screen resolution. While at it, remove some of the
unnecessary structures and only keep the resolution in one location.

Signed-off-by: Thierry Reding <thierry.reding@avionic-design.de>
Enable texture coordinates for FBO rendering

When rendering to an FBO, enable the texture coordinates to make sure
the fragment shader gets properly interpolated coordinates. This is
required to make fragment shaders other than a uniform color generator
work.

Signed-off-by: Thierry Reding <thierry.reding@avionic-design.de>
Generate checkerboard pattern

Instead of generating a uniform red "pattern", generate a checkerboard
pattern. This is in preparation for testing geometric transformations.

Signed-off-by: Thierry Reding <thierry.reding@avionic-design.de>