video: truetype: Add a scratch buffer to use malloc() less

The stb_truetype library performs around 5 allocations per character
rendered, totalling approximately 26KB of temporary memory. This creates
significant malloc/free overhead and heap fragmentation.

Add a scratch buffer mechanism that pre-allocates memory once during
probe and reuses it for each character. The buffer is reset at the start
of each putc_xy() call, and allocations come from this buffer using a
simple bump allocator with 8-byte alignment.

If the scratch buffer is exhausted (e.g. for very complex glyphs), the
allocator falls back to malloc transparently.

The scratch buffer is controlled by two new Kconfig options:
- CONSOLE_TRUETYPE_SCRATCH: Enable/disable the feature (default y)
- CONSOLE_TRUETYPE_SCRATCH_SIZE: Buffer size in bytes (default 32KB)

Co-developed-by: Claude <noreply@anthropic.com>
Signed-off-by: Simon Glass <simon.glass@canonical.com>

doc/usage/cmd/font.rst

@@ -85,6 +85,15 @@ CONFIG_CONSOLE_TRUETYPE_GLYPH_BUF enables a pre-allocated buffer for glyph
 rendering, avoiding malloc/free per character. The buffer starts at 4KB and
 grows as needed via realloc().
 
+CONFIG_CONSOLE_TRUETYPE_SCRATCH enables a scratch buffer for internal stbtt
+allocations. Without this, the TrueType library performs around 5 allocations
+per character (totalling ~26KB), creating malloc/free overhead and heap
+fragmentation. With the scratch buffer, memory is allocated once at probe time
+and reused for each character. CONFIG_CONSOLE_TRUETYPE_SCRATCH_SIZE sets the
+buffer size (default 32KB), which is sufficient for most Latin characters.
+Complex glyphs (CJK, emoji) or very large font sizes may need 64KB or more.
+Allocations exceeding the buffer size fall back to malloc transparently.
+
 CONFIG_VIDEO_GLYPH_STATS enables tracking of glyph-rendering statistics.
 
 Return value