general: Add setting shader_backend

GLASM is getting good enough that we can move it out of advanced
graphics settings. This removes the setting `use_assembly_shaders`,
opting for a enum class `shader_backend`. This comes with the benefits
that it is extensible for additional shader backends besides GLSL and
GLASM, and this will work better with a QComboBox.

Qt removes the related assembly shader setting from the Advanced
Graphics section and places it as a new QComboBox in the API Settings
group. This will replace the Vulkan device selector when OpenGL is
selected.

Additionally, mark all of the custom anisotropic filtering settings as
"WILL BREAK THINGS", as that is the case with a select few games.