Take a closer look at the various MR_OUTLINE flags

There are three:

```
#define MR_SHOW_OUTLINE (1<<0) // Draw the object in outline mode. Color specified by model_set_outline_color
#define MR_SHOW_OUTLINE_PRESET (1<<14) // Draw the object in outline mode. Color assumed to be set already.
#define MR_SHOW_OUTLINE_HTL (1<<26) // Show outlines (wireframe view) using HTL method
```

If we're using HTL exclusively, the first and third flags should be merged. After that's done, someone familiar with the model code should take a look at the `is_outlines_only_htl` and `is_outlines_only` variables.

I noticed this when reviewing some instances of `if (is_outlines_only_htl || (!is_outlines_only))` after the -nohtl removal in PR #641. On first glance this looks like it devolves to `if (outlines || !outlines)` and then to `if (true)`, but it might be something more nuanced.