OpenGL integration test.

Fixes #6
Metadata of program uniform variables.
Uniform metadata (name and type).
Check if uniform exists by name.
get_uniform_index method of program
variable_info -> uniform_info
Set uniform values.
OpenGL uniform variable handling (incomplete).
Re-shape image_loader interface.
Text loader.
Trace added, and test modified.
Work in progress: uniform variable integration.
asset_not_found shall be thrown if an asset cannot be resolved
Text loader must throw asset_not_found if asset is not found.
Copy resource files needed for tests.
Remove compiler warning.
Delete obsolete build system remains.
Do not throw exceptions in scene rendering.
Better diagnostic for link errors.
Correct resource paths.