Add support for single-component textures (uncompressed)

Loading of uncompressed single-component (which can be described
as grayscale, red-only, or luminance) textures from a PNG file is
now supported, including uploading to video memory. This should bring
performance improvement when single component textures are used.
Support for single-component texture compression formats still has
to be completed.

Also add support for the loading into system memory, but not
uploading to video memory, of 16-bit color component textures. This
is utilized in demo4(b) to allow for an elevation map with higher
precision (which is used to generate the terrain mesh, and not
directly used on the GPU).

Signed-off-by: Harm Hanemaaijer <fgenfb@yahoo.com>
Add more flexibility for the elevation map in demo4

The elevation map used in demo4 for Earth terrain can now have
higher (16-bit) precision. The configuration of the mesh detail
level has also been simplified.

Signed-off-by: Harm Hanemaaijer <fgenfb@yahoo.com>
Improve texture support; add HIDDEN object flag

Texture support has been improved, in particular for single
component RGTC1 (BC4) compressed textures (DDS format).

Add the SRE_OBJECT_HIDDEN object flag for objects that should not
be drawn but otherwise can be moved, be part of physics, or cast
shadows.

Signed-off-by: Harm Hanemaaijer <fgenfb@yahoo.com>
Improve Earth rendering demo (demo4/demo4b)

Allow more detailed elevation map to be used; support 16384x8192
textures which facilitates use of mip-maps.

Optionally hide the spacecraft in the middle of the screen (default:
hidden).

In demo4b (choreagraphed animation), use great circles for fly-overs,
giving much more flexibility for the chosen path.

Signed-off-by: Harm Hanemaaijer <fgenfb@yahoo.com>