Added an extruded convex hull as a raft instead of a grey plane.

- New shader transparent_object.shader
- Raft thickness is calculated in BuildVolume and used in
  ConvexHullDecorator, notified by a Signal.
- Removed old grey plane from BuildVolume.
- Vertex data below build plane is no longer discarded (caused convex
  hulls that are too small).
- Uses new functions in MeshBuilder (update Uranium as well).

CURA-1707

resources/shaders/transparent_object.shader

@@ -0,0 +1,74 @@
+[shaders]
+vertex =
+    uniform highp mat4 u_viewProjectionMatrix;
+    uniform highp mat4 u_modelMatrix;
+    uniform highp mat4 u_normalMatrix;
+
+    attribute highp vec4 a_vertex;
+    attribute highp vec4 a_normal;
+    attribute highp vec2 a_uvs;
+
+    varying highp vec3 v_vertex;
+    varying highp vec3 v_normal;
+    varying highp vec2 v_uvs;
+
+    void main()
+    {
+        vec4 world_space_vert = u_modelMatrix * a_vertex;
+        gl_Position = u_viewProjectionMatrix * world_space_vert;
+
+        v_vertex = world_space_vert.xyz;
+        v_normal = (u_normalMatrix * normalize(a_normal)).xyz;
+
+        v_uvs = a_uvs;
+    }
+
+fragment =
+    uniform mediump vec4 u_ambientColor;
+    uniform mediump vec4 u_diffuseColor;
+    uniform highp vec3 u_lightPosition;
+    uniform highp vec3 u_viewPosition;
+    uniform mediump float u_opacity;
+    uniform sampler2D u_texture;
+
+    varying highp vec3 v_vertex;
+    varying highp vec3 v_normal;
+    varying highp vec2 v_uvs;
+
+    void main()
+    {
+        // Copied from platform.shader, removed texture
+        mediump vec4 final_color = vec4(0.0);
+
+        /* Ambient Component */
+        final_color += u_ambientColor;
+
+        highp vec3 normal = normalize(v_normal);
+        highp vec3 light_dir = normalize(u_lightPosition - v_vertex);
+
+        /* Diffuse Component */
+        highp float n_dot_l = clamp(dot(normal, light_dir), 0.0, 1.0);
+        final_color += (n_dot_l * u_diffuseColor);
+
+        final_color.a = u_opacity;
+
+        gl_FragColor = final_color;
+    }
+
+[defaults]
+u_ambientColor = [0.3, 0.3, 0.3, 1.0]
+u_diffuseColor = [1.0, 1.0, 1.0, 1.0]
+u_opacity = 0.5
+u_texture = 0
+
+[bindings]
+u_viewProjectionMatrix = view_projection_matrix
+u_modelMatrix = model_matrix
+u_normalMatrix = normal_matrix
+u_lightPosition = light_0_position
+u_viewPosition = camera_position
+
+[attributes]
+a_vertex = vertex
+a_normal = normal
+a_uvs = uv0