feat: Points (#82)

Replaces the former sole Element entity by two, Sphere and Point, both implementing an Element trait. Adds Point display, uses the former Element display for Sphere. Adds a new "canned" configuration, and the ability to add, select, and nudge Point entities.

Co-authored-by: Aaron Fenyes <aaron.fenyes@fareycircles.ooo>
Reviewed-on: StudioInfinity/dyna3#82
Co-authored-by: Vectornaut <vectornaut@nobody@nowhere.net>
Co-committed-by: Vectornaut <vectornaut@nobody@nowhere.net>

app-proto/src/point.vert

@@ -0,0 +1,24 @@
+#version 300 es
+
+in vec4 position;
+in vec3 color;
+in float highlight;
+in float selected;
+
+out vec3 point_color;
+out float point_highlight;
+out float total_radius;
+
+// camera
+const float focal_slope = 0.3;
+
+void main() {
+    total_radius = 5. + 0.5*selected;
+    
+    float depth = -focal_slope * position.z;
+    gl_Position = vec4(position.xy / depth, 0., 1.);
+    gl_PointSize = 2.*total_radius;
+    
+    point_color = color;
+    point_highlight = highlight;
+}