Crabby only supports point sampling

[mainroach]

Currently the block sampling method that crabby uses only supports POINT sampling mode for filtering.
Some 2D games use filtering everywhere, as part of their art-style. We need to find a way to support this use case.

[mainroach]

A potential solution to this problem is to pad each of the 4x4 blocks with a 2pixel border (from the source image) and adjust the pixel-sampling-location in the shader to take into account the new sample positions. This will allow bilinear sampling for the 4x4 block and remove some of the extra artifacts.