I have a Raspberry-Pi with Minecraft-Pi. I want to update the rendering-engine a bit. Eg. adding mip-maps and animations like an Optifine for Minecraft-Pi. Does anyone know a link to the C++-sourcecode?
Maybe you've realized this by now, but what you linked is the binary, not the source code.
For anyone else looking to reverse-engineer and extend Pi Edition, this thread has several examples of patches for v0.1.1 and the pre-release. The mods range from a speed hack to anaglyph rendering. Furthermore, Phirel gives some great advice in the survival mode patch thread. The tip of comparing against Pocket Edition (0.6.1) symbols is related to what @yellowwinner said above.
Adding support for mipmaps might be more challenging than any existing Pi Edition patches to date, but with enough effort it's possible even without source code.