feat(render): style, GLSL line/text pipelines, staging batches, bitmap font

include/plot/render/text.hpp

@@ -0,0 +1,32 @@
+#pragma once
+#include <plot/core/types.hpp>
+
+namespace plot::render {
+
+/// Text metrics the plot layer lays axes and labels out with. It is a pure
+/// interface so the layout code never depends on the bitmap font, and so a
+/// caller can measure with a host toolkit's font instead.
+///
+/// Text is passed as UTF-8, not as a byte-per-character string: `plot::format_si`
+/// emits U+00B5, U+00B0 and U+03A9 for micro, degree and ohm, and an
+/// implementation that ignores that prints mojibake.
+class TextBackend {
+ public:
+  TextBackend() noexcept = default;
+  TextBackend(const TextBackend&) = default;
+  TextBackend& operator=(const TextBackend&) = default;
+  virtual ~TextBackend() = default;
+
+  /// Width of `utf8` in the same units as `height()` (pixels for the bitmap
+  /// font). Zero for the empty string.
+  virtual f32 measure(const char* utf8) const = 0;
+
+  /// Line height: the cell a single line of text occupies.
+  virtual f32 height() const = 0;
+
+  /// Distance from the top of the cell to the baseline, so a caller can align
+  /// text bottoms instead of cell bottoms.
+  virtual f32 ascent() const = 0;
+};
+
+}  // namespace plot::render