Support for more component types in metadata textures

[mzschwartz5]

Current state:

For the structural metadata extension, texels can encode metadata via property textures. Currently, the only supported metadata property type is UINT8. A texture can encode a (fixed-size) array of uints (interpreted in the shader as ivecN), or normalized uints (interpreted as floats), but it cannot use multiple channels to represent higher-precision numbers.

Desired state:

Represent higher precision ints and floats (up to 32 bits - using all 4 channels of each texel), as well as bools and smaller fixed-size arrays of higher precision (e.g. ivec2 with half-precision). Strings and dynamic arrays will not be supported still.

The underlying texture will still be RGB(A)8 format, but the data from each channel will be bit-shifted to form a higher precision format.

Some misc. notes:

• The structural metadata spec states:

For unsigned integer component types, values are normalized between [0.0, 1.0]. For signed integer component types, values are normalized between [-1.0, 1.0]

Currently, since we only support the UINT8 component type, we only consider the first case here. We'll need to also consider normalized signed integer types going forward.

• webgl 1 does not support uint types in glsl