[lang] Texture support

[bobcao3]

We wish to support hardware texture & image load-store. In the JIT mode (Python), we will provide a new class called Texture

You can create a texture, transfer data from a PIL image, numpy array, or NDArray (or other potential in-memory image formats), and the textures can be passed into Taichi kernels for use.

Initially we will support:

• 2D Textures (1~4 channels)
• Image data from NDArray
• LOD0
• Normalized texture formats (internal int, read out float)
• FP16 / FP32 texture formats (internal ieee754 FP)
• Passing textures as kernel arguments, similar to NDArrays
• Bilinear explicit LOD sampling
• Implicit LOD sampling with gradients (textureGrad)
• Texel fetching (texelFetch)
• Texel quad gathering (textureGather)

Future plans:

• 1D / 3D Textures
• PIL / numpy / tensor bidirectional image IO
• LODs & LOD generation helpers
• Sampler parameter control
• Integer textures (Corresponds to isamplerND / usamplerND in GLSL)
• AOT support
• Image load-store
• Texture array (Descriptor indexing / descriptor array, not sampler2DArray)

[k-ye]

Update: 06/30

Oops, OpenGL is also supported.

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Note that we are only supporting texture on Vulkan backend as a start.

[lyd405121]

Will compressed format texture supported in the future?

• Like DXT1-7(windows),pvr(powervr),etc(all platform)
• FYI, Khronos group is working on a convertor or standard dealing with all kinds of format