feat: project skeleton, core types and math with transform tests

include/plot/math/mat3.hpp

@@ -0,0 +1,31 @@
+#pragma once
+#include <plot/core/types.hpp>
+
+namespace plot::math {
+
+/// Column-major 3x3 matrix of `f32`, the layout OpenGL expects for `mat3`
+/// uniforms.
+struct Mat3 {
+  f32 m[9] = {1.0f, 0.0f, 0.0f, 0.0f, 1.0f, 0.0f, 0.0f, 0.0f, 1.0f};
+
+  static constexpr Mat3 identity() noexcept { return Mat3{}; }
+
+  /// Column-major product `(*this) * rhs`.
+  constexpr Mat3 operator*(const Mat3& rhs) const noexcept {
+    Mat3 out{0.0f, 0.0f, 0.0f, 0.0f, 0.0f, 0.0f, 0.0f, 0.0f, 0.0f};
+    for (usize col = 0; col < 3; ++col) {
+      for (usize row = 0; row < 3; ++row) {
+        f32 acc = 0.0f;
+        for (usize k = 0; k < 3; ++k) acc += m[k * 3 + row] * rhs.m[col * 3 + k];
+        out.m[col * 3 + row] = acc;
+      }
+    }
+    return out;
+  }
+};
+
+}  // namespace plot::math
+
+namespace plot {
+using math::Mat3;
+}