implement the complete stroke shader with custom blending mode

engine/brush.cpp

@@ -75,7 +75,7 @@ bool ui::BrushMesh::create()
         { {  .5f, -.5f, 0, 1 }, { 1, 0 } }, // D       A----D
     };
     glGenBuffers(3, buffers);
-    if (!buffers)
+    if (!(buffers[0] && buffers[1] && buffers[2]))
         return false;
 
     static instance_t inst{ glm::mat4(), .1f };
@@ -170,6 +170,7 @@ ui::StrokeSample ui::Stroke::randomize_sample(const glm::vec2& pos, float pressu
 }
 std::vector<ui::StrokeSample> ui::Stroke::compute_samples()
 {
+    if (m_keypoints.empty()) return {};
     int nsamples = (int)glm::floor((m_keypoints.back().dist - m_dist) / m_step);
     std::vector<StrokeSample> samples;
     samples.reserve(nsamples); // preallocate the estimate number of samples