LibGL+LibSoftGPU: Move rendering related code to LibSoftGPU library

This introduces a new library, LibSoftGPU, that incorporates all
rendering related features that formerly resided within LibGL itself.

Going forward we will make both libraries completely independent from
each other allowing LibGL to load different, possibly accelerated,
rendering backends.

Userland/Libraries/LibSoftGPU/Clipper.cpp

@@ -0,0 +1,83 @@
+/*
+ * Copyright (c) 2021, Jesse Buhagiar <jooster669@gmail.com>
+ * Copyright (c) 2021, Stephan Unverwerth <s.unverwerth@serenityos.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#include <AK/Format.h>
+#include <AK/ScopeGuard.h>
+#include <LibSoftGPU/Clipper.h>
+
+namespace SoftGPU {
+
+bool Clipper::point_within_clip_plane(const FloatVector4& vertex, ClipPlane plane)
+{
+    switch (plane) {
+    case ClipPlane::LEFT:
+        return vertex.x() >= -vertex.w();
+    case ClipPlane::RIGHT:
+        return vertex.x() <= vertex.w();
+    case ClipPlane::TOP:
+        return vertex.y() <= vertex.w();
+    case ClipPlane::BOTTOM:
+        return vertex.y() >= -vertex.w();
+    case ClipPlane::NEAR:
+        return vertex.z() >= -vertex.w();
+    case ClipPlane::FAR:
+        return vertex.z() <= vertex.w();
+    }
+
+    return false;
+}
+
+GL::GLVertex Clipper::clip_intersection_point(const GL::GLVertex& p1, const GL::GLVertex& p2, ClipPlane plane_index)
+{
+    // See https://www.microsoft.com/en-us/research/wp-content/uploads/1978/01/p245-blinn.pdf
+    // "Clipping Using Homogeneous Coordinates" Blinn/Newell, 1978
+
+    float w1 = p1.position.w();
+    float w2 = p2.position.w();
+    float x1 = clip_plane_normals[plane_index].dot(p1.position);
+    float x2 = clip_plane_normals[plane_index].dot(p2.position);
+    float a = (w1 + x1) / ((w1 + x1) - (w2 + x2));
+
+    GL::GLVertex out;
+    out.position = p1.position * (1 - a) + p2.position * a;
+    out.color = p1.color * (1 - a) + p2.color * a;
+    out.tex_coord = p1.tex_coord * (1 - a) + p2.tex_coord * a;
+    return out;
+}
+
+void Clipper::clip_triangle_against_frustum(Vector<GL::GLVertex>& input_verts)
+{
+    list_a = input_verts;
+    list_b.clear_with_capacity();
+
+    auto read_from = &list_a;
+    auto write_to = &list_b;
+
+    for (size_t plane = 0; plane < NUMBER_OF_CLIPPING_PLANES; plane++) {
+        write_to->clear_with_capacity();
+        // Save me, C++23
+        for (size_t i = 0; i < read_from->size(); i++) {
+            const auto& curr_vec = read_from->at((i + 1) % read_from->size());
+            const auto& prev_vec = read_from->at(i);
+
+            if (point_within_clip_plane(curr_vec.position, static_cast<ClipPlane>(plane))) {
+                if (!point_within_clip_plane(prev_vec.position, static_cast<ClipPlane>(plane))) {
+                    auto intersect = clip_intersection_point(prev_vec, curr_vec, static_cast<ClipPlane>(plane));
+                    write_to->append(intersect);
+                }
+                write_to->append(curr_vec);
+            } else if (point_within_clip_plane(prev_vec.position, static_cast<ClipPlane>(plane))) {
+                auto intersect = clip_intersection_point(prev_vec, curr_vec, static_cast<ClipPlane>(plane));
+                write_to->append(intersect);
+            }
+        }
+        swap(write_to, read_from);
+    }
+
+    input_verts = *read_from;
+}
+}