Use a single program for drawing background images

kitty/bgimage_vertex.glsl

@@ -3,6 +3,13 @@
 #define top  1.0f
 #define right  1.0f
 #define bottom  -1.0f
+#define tex_left 0
+#define tex_top 0
+#define tex_right 1
+#define tex_bottom 1
+
+uniform float tiled;
+uniform vec4 sizes;  // [ window_width, window_height, image_width, image_height ]
 
 out vec2 texcoord;
 
@@ -12,10 +19,24 @@ const vec2 pos_map[] = vec2[4](
     vec2(right, bottom),
     vec2(right, top)
 );
+const vec2 tex_map[] = vec2[4](
+    vec2(tex_left, tex_top),
+    vec2(tex_left, tex_bottom),
+    vec2(tex_right, tex_bottom),
+    vec2(tex_right, tex_top)
+);
 
 
+float scale_factor(float window, float image) {
+    return max(1, window / image);
+}
+
+float tiling_factor(int i) {
+    return tiled * scale_factor(sizes[i], sizes[i + 2]) + (1 - tiled);
+}
+
 void main() {
-    vec2 vertex = pos_map[gl_VertexID];
-    texcoord = clamp(vec2(vertex[0], vertex[1]*-1), 0, 1);
-    gl_Position = vec4(vertex, 0, 1);
+    vec2 tex_coords = tex_map[gl_VertexID];
+    texcoord = vec2(tex_coords[0] * tiling_factor(0), tex_coords[1] * tiling_factor(1));
+    gl_Position = vec4(pos_map[gl_VertexID], 0, 1);
 }