implement vertex coordinate mapping in text.v.glsl

runtime/shaders/text.v.glsl

@@ -1,9 +1,16 @@
+uniform vec2 screen_size;
+uniform vec2 position;
+
 attribute vec4 coords;
 
 varying vec2 texture_coord_v;
 
 void main(void)
 {
-    gl_Position = vec4(coords.xy, 0, 1);
+    // Map coordinates such that:
+    // (-0.5 .. screen_size.x-0.5) => (-1.0 .. 1.0)
+    gl_Position = vec4(2.0 * (position.x + coords.x + 0.5) / screen_size.x - 1.0,
+                       2.0 * (position.y + coords.y + 0.5) / screen_size.y - 1.0,
+                       0, 1);
     texture_coord_v = coords.zw;
 }