groups: improved functionality

Various improvements and fixes to groups

app/shaders/opacity.frag

@@ -0,0 +1,11 @@
+// Inputs
+uniform sampler2D tex_in;
+uniform float opacity_in;
+
+// Input texture coordinate
+in vec2 ove_texcoord;
+out vec4 frag_color;
+
+void main() {
+  frag_color = texture(tex_in, ove_texcoord) * opacity_in;
+}