[ FF7 ] Add field and battle texture filtering options.

[zaphod77]

Summary

Original steam release allows you to apply bilinear filtering to the field and battle textures. This is impossible in FFNX as near as I can tell.

Basic example

enable_bilinear option exists, but only seems to affect the field models, as per the original pull request (moving trains in the trainyard, for example). The main low definition field images are still shown using nearest neighbor, and the battle textures are blocky still.

Motivation

Provide feature parity with the steam version. currently even with no mods, we need to use FFNX to get proper controller support for xinput controllers, which causes loss of the bilinear filter option that the original game has.