LibSoftGPU: Optimize `GPU::Vertex` lists

On my machine, benchmarking 3DFileViewer revealed ~2.5% of CPU time
spent in `Vector<GPU::Vertex>::try_append`. By carefully managing list
capacities, we can remove this method from profiles altogether.

Userland/Libraries/LibSoftGPU/Clipper.cpp

@@ -76,6 +76,10 @@ FLATTEN static void clip_plane(Vector<GPU::Vertex>& input_list, Vector<GPU::Vert
     if (input_list_size == 0)
         return;
 
+    // Ensure we can perform unchecked appends in the loop below
+    if (input_list_size * 2 > output_list.capacity())
+        output_list.ensure_capacity(input_list_size * 2);
+
     auto const* prev_vec = &input_list.data()[0];
     auto is_prev_point_within_plane = point_within_plane<plane>(*prev_vec, clip_plane);
 
@@ -84,10 +88,10 @@ FLATTEN static void clip_plane(Vector<GPU::Vertex>& input_list, Vector<GPU::Vert
         auto const is_curr_point_within_plane = point_within_plane<plane>(curr_vec, clip_plane);
 
         if (is_curr_point_within_plane != is_prev_point_within_plane)
-            output_list.append(clip_intersection_point<plane>(*prev_vec, curr_vec, clip_plane));
+            output_list.unchecked_append(clip_intersection_point<plane>(*prev_vec, curr_vec, clip_plane));
 
         if (is_curr_point_within_plane)
-            output_list.append(curr_vec);
+            output_list.unchecked_append(curr_vec);
 
         prev_vec = &curr_vec;
         is_prev_point_within_plane = is_curr_point_within_plane;

Userland/Libraries/LibSoftGPU/Device.cpp

@@ -766,6 +766,9 @@ Device::Device(Gfx::IntSize size)
 {
     m_options.scissor_box = m_frame_buffer->rect();
     m_options.viewport = m_frame_buffer->rect();
+
+    // Ensure we can always append 3 vertices unchecked
+    m_clipped_vertices.ensure_capacity(3);
 }
 
 GPU::DeviceInfo Device::info() const
@@ -1112,9 +1115,9 @@ void Device::draw_primitives(GPU::PrimitiveType primitive_type, Vector<GPU::Vert
     // Clip triangles
     for (auto& triangle : m_triangle_list) {
         m_clipped_vertices.clear_with_capacity();
-        m_clipped_vertices.append(triangle.vertices[0]);
-        m_clipped_vertices.append(triangle.vertices[1]);
-        m_clipped_vertices.append(triangle.vertices[2]);
+        m_clipped_vertices.unchecked_append(triangle.vertices[0]);
+        m_clipped_vertices.unchecked_append(triangle.vertices[1]);
+        m_clipped_vertices.unchecked_append(triangle.vertices[2]);
         m_clipper.clip_triangle_against_frustum(m_clipped_vertices);
 
         if (m_clip_planes.size() > 0)