Physics & main character rework

It's been a while!

This took a long time to get finished as I had to recode a big part of the character, which in turn led to needing redrawing, etc. Now I have better control over how the character works, and his collisions should be more consistent all around.

Changes : 

-the character can slide on corners now, making overall motion more fluid.

-the character's sprite has been redrawn to be at the correct size (so that pixels stay coherent). 

-semisolid platforms function better overall. 

-sharks are easier to evade. 

-small adjustments in level "The cage" to make it easier. 

-other small fixes and tweaks.