Codechange: Remove direct access to _colour_gradient.

Access is now through GetColourGradient, which ensures parameters are in range.

Codechange: Store Colours in Colours type. (#11625)

This reduces casts, some magic numbers, and introduces a bit of type-safety.

Codechange: allow certain enumeration to be added

Otherwise C++20 doesn't like it.

Codechange: Make all NWidgetPart arrays constexpr.

This ensures that the arrays are not created at runtime and prevents using non-constexpr values.

Codechange: Replace mishmash of types for widget index with WidgetID.

Indices were stored as int, but often passed around as uint/uint8_t and casts.

Now they should all use WidgetID.

Codechange: Use std::map to provide indexed widget access.

This removes the need to determine the biggest widget index and replaces C-style memory handling.

Codechange: Remove FONT_HEIGHT_... macros. (#11481)

These make it look like we're dealing with a constant, but actually each is a call to `GetCharacterHeight(...)`.

Fix #9545: Crash when all cargo types are disabled. (#11432)

This is not a very useful state, but it's nice to not crash.

Some parts of the game don't (yet) check for cargo types being redefined, that is out-of-scope here.

Codechange: Add __FILE__/__LINE__ of WindowDesc to WindowDesc.

This is to allow unit-tests to produce more useful output.

Codechange: Move includes to correct place.

Some other source file inadvertently included things via smallmap_gui.h

Codechange: Use begin/end of nwidget parts of begin/length.

This simplifies processing nwidget parts as, unlike the remaining length, the pointer to the end of the list never changes. This is the same principle as we use(d) for tracking end instead of length for C-style strings.

And this removes 160~ instances of the lengthof() macro.

Codechange: automatic adding of _t to (u)int types, and WChar to char32_t

for i in `find src -type f|grep -v 3rdparty/fmt|grep -v 3rdparty/catch2|grep -v 3rdparty/opengl|grep -v stdafx.h`; do sed 's/uint16& /uint16 \&/g;s/int8\([ >*),;[]\)/int8_t\1/g;s/int16\([ >*),;[]\)/int16_t\1/g;s/int32\([ >*),;[]\)/int32_t\1/g;s/int64\([ >*),;[]\)/int64_t\1/g;s/ uint32(/ uint32_t(/g;s/_uint8_t/_uint8/;s/Uint8_t/Uint8/;s/ft_int64_t/ft_int64/g;s/uint64$/uint64_t/;s/WChar/char32_t/g;s/char32_t char32_t/char32_t WChar/' -i $i; done

Fix: Integer overflow in LinkGraphOverlay::ShowTooltip for long links

In particular when fully zoomed in
This could result in tooltips not being shown at all, or being shown
when not appropriate

Change: Cargo flow legend only shows defined cargo. (#10872)

This removes the large mostly-empty grid of cargo buttons when not using a complex industry set.

Fix: Linkgraph legend assumes strings are small.

Ensure all linkgraph legend strings are drawn with FS_SMALL forced, so
that it does not rely on NewGRF authors.

Codechange: introduce GetMainWindow() to properly account for nullptr checks

Some nullptr checks have been removed as they were not triggered with nullptr
with the null video driver and in dedicated server mode.

Change: Use Rect helpers for widget drawing.

This replaces repetitive and sometimes unwieldy use of constants.

Change: Use CenterBounds function in more places.

CenterBounds may have a 1 pixel difference so the result is not exactly
the same.

Codechange: Implement OnTooltip event for custom window tooltips.

This avoids windows from needing to know or care about tooltip delay settings.

Fix #7004: Mark linkgraph dirty to be rebuilt on next draw call. (#7265)

Previously the linkgraph was rebuilt before the viewport extents were finalized.

Codechange: [Linkgraph GUI] Replace line visibility detection algorithm

Use an implementation of the Cohen-Sutherland line-clipping algorithm.
The previous algorithm had an excessive false-positive rate.
Line-rendering is sufficiently expensive that using a line-clipping
algorithm with a much lower false-positive rate is a net performance
benefit.

Add: Hover tool-tips to cargo dest flow legend window.

This is to improve the usability of the window.
The two-letter abbreviations are not always clear, in particular
when using a large number of cargoes.
The company colours can be ambiguous when there are a large
number of companies.

Fix: Poor contrast in cargo dest flow legend window cargo labels.

Select foreground colour depending on the brightness of the background.

Previously all cargo labels were rendered using black text, even
the background cargo colour was dark/black.
As an example: FIRS coal was black text on a black background.