Codechange: use initializer_lists for the settings tables

Not using vectors as those require copying from the initializer list and that
makes unique_ptrs to the actual SettingDesc objects later impossible.

Cleanup: remove and/or fix some confusing comments

The comments for SettingDescType; it is a byte, so not 4 bytes and since it is not a flag there are about 250 other possibilities left instead of 9.
SettingGuiFlag is uint16 so has 2 bytes allocated.
SettingDescGlobVarList and related comments imply that global vars cannot be used elsewhere, but they are used for settings just fine. Even then the type is not used anywhere else but the definition of the table.

Feature: allow a toggle to enable/disable vsync

Vsync should be off by default, as for most players it will be
better to play without vsync. Exception exist, mainly people who
play in fullscreen mode.

Add: Option to (dis-)allow accelerated video drivers. (#8819)

The video drivers using the OpenGL backend are currently our only
accelerated drivers. The options defaults to off for macOS builds and
to on everywhere else.

Co-authored-by: Michael Lutz <michi@icosahedron.de>

Codechange: validate that "max" value of settings fit in their storage

This is an easy mistake to make, so protect us against making such
mistakes, by validating it doesn't happen.

Codechange: Don't use cpp_offsetof in the save/load code.

Many of the member variables that are used in save/load are inside types
that are not standard layout types. Using pointer arithmetics to determine
addresses of members inside types that are not standard layout is generally
undefined behaviour. If we'd use C++17, it is conditionally supported, which means
each compiler may or may not support it. And even then using it for individual
array elements is syntactically not supported the the standard offsetof function.

Unfortunately, the trickery employed for saving linkgraph settings causes quite some
clutter in the settings ini files.

Codechange: Refactor the Freetype font cache into a generic part that does not depend on Freetype and into one that does.

This makes it easier to add other TrueType font rendering engines.

Feature: Add option to adjust font size separately from GUI size. (#7003)

Adds an option in the "Game Options" next to "Interface Size" called "Font Size". Available options are normal, double, and quad.

Codechange: Convert saveload numbers to enum values.

(This was mostly achieved with a few in-place regexes)

(svn r26522) -Add: A config-file-only setting to disable usage of 8bpp video modes.
-Remove: [win32] fullscreen_bpp setting, which is replaced by above setting.
-Change: Disable usage of 8bpp blitters and video modes by default. Many modern OS and hardware cause issues with those.