demos/triangle: Add a demo program.

demos/triangle.vert

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+#version 150
+#extension GL_ARB_separate_shader_objects : enable
+
+layout(location = 0) in vec4 position_in;
+layout(location = 1) in vec4 colour_in;
+
+layout(location = 0) out vec4 colour_out;
+
+void main(void)
+{
+    gl_Position.xzw = position_in.xzw;
+    gl_Position.y = -position_in.y;
+    colour_out = colour_in;
+}