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

include/plot/render/shaders.hpp

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+#pragma once
+
+namespace plot::render {
+
+/// GLSL source of the instanced antialiased line pipeline.
+///
+/// The vertex shader takes one segment per instance (`a_p0` at location 0,
+/// `a_p1` at location 1, both with a vertex attribute divisor of 1), expands it
+/// in pixel space along its normal by `u_width * 0.5 + 1.0` pixels of AA fringe,
+/// and emits a four-vertex `GL_TRIANGLE_STRIP` quad whose corners come from
+/// `gl_VertexID` (0 and 1 are the two sides of `a_p0`, 2 and 3 of `a_p1`). It
+/// outputs `v_dist`, the signed distance from the segment axis in pixels, and
+/// `v_along`, the distance travelled from `a_p0` in pixels.
+///
+/// Uniforms: `u_scale` (vec2, data space to pixels), `u_offset` (vec2, pixels),
+/// `u_viewport` (vec2, pixels), `u_width` (float, pixels), `u_color` (vec4).
+/// `plot::Rect` is top-left origin while GL is bottom-left, so the y axis is
+/// flipped exactly once, here.
+///
+/// The fragment shader discards outside the fringe and ramps alpha over the last
+/// pixel of the half width with `smoothstep`, which is what keeps a 1px trace
+/// from aliasing into dashes.
+extern const char* const kLineVS;
+extern const char* const kLineFS;
+
+/// GLSL source of the instanced text pipeline.
+///
+/// One glyph cell per instance: `a_pos` at location 0 is the quad's top-left
+/// corner in Rect pixels and `a_uv` at location 1 is the glyph's atlas texel
+/// origin, both with a vertex attribute divisor of 1. The quad grows by
+/// `u_px_range` pixels per cell and the fragment shader samples the
+/// single-channel `GL_R8` atlas at the nearest texel centre, so `GL_NEAREST`
+/// never lands on a cell boundary. `u_color` tints the straight alpha.
+///
+/// Uniforms: `u_viewport` (vec2, pixels), `u_px_range` (vec2, pixels per glyph
+/// cell), `u_tex_size` (vec2, atlas texels), `u_atlas` (sampler2D, unit 0),
+/// `u_color` (vec4).
+extern const char* const kTextVS;
+extern const char* const kTextFS;
+
+}  // namespace plot::render