Cleanup a bunch of shader infrastructure

1) No longer us glScissor. It's an awful API and is not available in
   Vulkan. Instead the graphics drawing code ensures the graphic is
   drawn within the current viewport

2) Use generated code to automatically get the locations of uniforms
   from shaders. Greatly simplifies adding new uniforms to a shader.

3) Dont use a VAO for loading graphics vertices. Greatly simplifies
   a bunch of book keeping code.

kitty/graphics.h

@@ -76,7 +76,7 @@ typedef struct {
 } BackgroundImage;
 
 typedef struct {
-    float vertices[16];
+    ImageRect src_rect, dest_rect;
     uint32_t texture_id, group_count;
     int z_index;
     id_type image_id;