Fix for RenderTexture stretching/low-quality when creating a RenderTextu...

Fix for RenderTexture stretching/low-quality when creating a RenderTexture of size larger than screen size.

Added a new Create/Init function to RenderTexture with width/height parameters in pixels (instead of points). Does not change behavior of current create/init functions.

Needed for screen-size-independent RenderTexture processing like generating dynamic textures, or PingPong rendering etc...