libavg just got another major optimization.

I implemented an image registry and cache for libavg. ImageNodes that reference the same image file now reference the same bitmap in CPU memory and the same texture in GPU memory. This is completely hidden from the app developer, who just specifies the file location for all instances. The obvious benefit is that this saves a lot of memory if an application re-uses lots of bitmaps. The less obvious benefit is that it speeds things up as well: avg_checkspeed, which tests with thousands of identical ImageNodes, can now handle around 15000 Nodes at 60 FPS on my old i7 (still Core i7 920 Bloomfield, 2.66 MHz, NVidia GF260 like in the old benchmarks). This is twice as many as before.