Shaders: Config UI desyncs with shader backend

[DrFlarp]

Steps to reproduce:

1. Set number of shaders to 0
2. change any value in Shader 1
3. Set number of shaders to 1

Result: shader engine does not reflect the changes made when the shader was inactive.

This leads to quite a bit of confusion when minarch is launched with no shader options set in the config files, since the shader engine's defaults differ from the UI's. (for example, shader backend scale = 1 but UI default scale = 2). User has to wiggle every option back and forth in order to see which one isn't set properly.

@frysee should I work on this as a part of #679 or as its own PR? (I expect this one to be a smaller fix)

EDIT: should also mention that one of the offending lines is generic_video.c:618