Merge pull request #6 from bvssvni/master

Added `Texture::from_rgba8`
Merge pull request #8 from bvssvni/master

Added `get_size`, `get_width` and `get_height` to `Texture`
Merge pull request #10 from bvssvni/master

Implemented `texture::FromMemoryAlpha`
Made `from_memory_alpha` a normal method