refactor(render): TextBackend owns its atlas, and staged glyphs carry a rotation basis

include/plot/render/shaders.hpp

@@ -30,16 +30,24 @@ 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
+/// One glyph cell per instance, every attribute with a vertex attribute divisor
+/// of 1: `a_pos` at location 0 is the cell's top-left corner in Rect pixels,
+/// `a_uv` at location 1 is the glyph rect's normalised top-left in the atlas and
+/// `a_axis` at location 2 is the cell's local +x axis, a unit vector in pixels.
+///
+/// The vertex shader builds the cell from that basis and its perpendicular --
+/// the cell's own +v direction -- so `a_axis` of `(0, -1)` draws a run that
+/// advances up the screen, and it maps `a_uv` over the cell with the same
+/// handedness, so a glyph's top row stays its top row however the cell is
+/// turned. The fragment shader converts back to texels and 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).
+/// Uniforms: `u_viewport` (vec2, pixels), `u_px_range` (vec2, the glyph cell's
+/// size on screen in pixels), `u_glyph_uv` (vec2, the glyph cell's size in
+/// normalised atlas coordinates -- one size for the whole draw, since every
+/// glyph of a frame comes from one atlas), `u_tex_size` (vec2, atlas size in
+/// texels), `u_atlas` (sampler2D, unit 0), `u_color` (vec4).
 extern const char* const kTextVS;
 extern const char* const kTextFS;