use a geometry shader to render rectangles

res/sh.geom.glsl

@@ -0,0 +1,25 @@
+#version 330 core
+layout(points) in;
+layout(triangle_strip, max_vertices = 4) out; // output the vertices of this rectangle
+
+void push_vert(float x, float y) {
+	gl_Position = vec4(x, y, 0.0F, 1.0F);
+	EmitVertex();
+}
+
+void main() {
+	// position data is stored as (x,y,w,h)
+	vec4 rect_a = gl_in[0].gl_Position; // get the rectangle data which will be our first point of the rectangle
+	vec2 rect_d = vec2(                 // compute point D
+		rect_a.x + rect_a.z,
+		rect_a.y - rect_a.w);
+
+	// A---B
+	// C---D
+	// triangle_strip will use the last two vertices to create a triangle
+	push_vert(rect_a.x, rect_a.y); // A
+	push_vert(rect_d.x, rect_a.y); // B
+	push_vert(rect_a.x, rect_d.y); // C
+	push_vert(rect_d.x, rect_d.y); // D
+	EndPrimitive();
+}