Check error code from error_open_log()

Signed-off-by: Paul Holden <paulholden2@gmail.com>
Always write to engine log file

Signed-off-by: Paul Holden <paulholden2@gmail.com>
Add FILE_OPEN_FAIL error code

Signed-off-by: Paul Holden <paulholden2@gmail.com>
Return error code from engine_open_log()

Signed-off-by: Paul Holden <paulholden2@gmail.com>
Add engine_chk_sysreq and associated error codes

- Checks if the system meets the engine's minimum requirements to run
  properly, e.g. minimum OpenGL version.

Signed-off-by: Paul Holden <paulholden2@gmail.com>
Add engine_chk_args() to set engine options

- Also added NO_LOG_FILE engine option. Uses stdout as the engine log,
  useful for debugging with a console.

Signed-off-by: Paul Holden <paulholden2@gmail.com>
Clean-up on heightmap code.

- Variable renaming, removing redundant code, structs, fields, etc.

Signed-off-by: Paul Holden <paulholden2@gmail.com>