New setting: Texture replacement load speed

[hrydgard]

This:

• Allows more time per frame for loading replacement textures if frame rate is lower
• Adds a setting that lets you control the tradeoff between smoothness and stutter when loading replacement textures
• Optimizes ini file loading a bit
• Avoids reloading the ini file unnecessarily

Should address the complaints of texture replacements not all loading at once - now you can get that behavior if you want it, but it'll likely cause stutters just like it does in old builds that didn't use a frame budget (or failed to use it properly).