According to Erik @ NVidia the open source NVidia driver will not
create a EGLImage from a DMABUF if the target is not
GL_TEXTURE_EXTERNAL_OES. This change set converts the dmabuf texture
from GL_TEXTURE_2D to GL_TEXTURE_EXTERNAL_OES and at runtime performs a
global search & replace on fragment shaders as needed to remain
compatible, replacing `sampler2D` with `samplerExternalOES`.
Ref: https://github.com/NVIDIA/open-gpu-kernel-modules/discussions/243#discussioncomment-3283415
This fixes a rounding issue on certain hardware (NVidia) which actually
implement mediump as half precision (16-bit) float. It's safe to assume
`highp` is available as if the GPU does not support it, then the shader
compiler will try to find a lower precision that is supported by the GPU
Using 4x4 means that some pixels will be outside of the lanczos window. The
ideal lanczos function should in fact be zero in those areas, so we shouldn't
waste time processing those pixels.
I can't notice any difference in the results.
