rebase: renderer rewrite glyph cache (lite-xl/lite-xl#1845)

* renderer: rewrite glyph cache

This commit splits the current GlyphSet-based system into 2 caches,
CharMap and GlyphMap, which maps codepoints -> glyph IDs and glyph IDs -> glyphs respectively.
Each GlyphMap contains a number of GlyphAtlas mapped by their width,
and each GlyphAtlas has a list of surfaces with the same width.
Surfaces within the GlyphAtlas acts like a bump allocator, but is flexible
enough to allow older GlyphAtlas surfaces to be recycled.

GlyphMetric now contains atlas_idx and surface_idx, which points to the correct surface.
Since GlyphAtlas supports rudimentary packing & allocation, we no longer render whole
GlyphSets and this speeds up rendering in most cases.

Tab width is no longer set with the hacky "modify the GlyphMetric" hack.

* renderer: set natlas to 0 after freeing

* renderer: fix printf warning

* renderer: fix memory leak

* renderer: better whitespace rendering

* renderer: fix ubsan warning when casting ints

* renderer: fix tab handling

There's a bug with inconsistent tab widths caused by fontgroups.

* renderer: fix glyphs being loaded over and over

* renderer: add glyphmap size

* renderer: store per-surface offset_y and use it to find best-fitting surface

* renderer: fix MSVC compiler error

* renderer: remove return value from ren_font_glyph

* renderer: refactor xadvance calculation

* renderer: fix double free SDL_RWops if FT_Set_Pixel_Size fail

* renderer: always try .notdef before U+25A1

* renderer: disable ren_font_dump yet again

Accidentally commited this change.

* renderer: remove unused imports

* renderer: fix double free with FT_Open_Face

* renderer: return SDL_Surface in font_find_glyph_surface

* renderer: bring back metric flags for future extension

* renderer: refactor xadvance calculation into macro

* renderer: fix comment

* renderer: store GlyphMetric directly in the surface

* renderer: remove duplicated comment

* renderer: rename font_find_glyph_surface to font_allocate_glyph_surface

* renderer: refactor glyphmetric retrieval into an inline function

* renderer: do not render glyphs with bitmap set to null

This is a weird edge case, but at least it shouldn't crash

* renderer: refactor face metric code into its own function

* renderer: actually check if glyph fits in surface

* renderer: rudimentary support for non-scalable faces

At least it won't render nothing on the screen

* renderer: check for font_surface instead of metric directly

This is safe and shorter

* renderer: fix indentation

* renderer: rename GLYPH_PER_ATLAS to GLYPHS_PER_ATLAS

* renderer: rename all GLYPH_PER_ATLAS correctly

* renderer: make utf8_to_codepoint slightly more durable

* renderer: fix compiler unsigned cast warning

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