tweak attrs: fix UV attribute output type.

Materials: support the custom uniform attributes in Eevee.

The attributes are provided to the shader via a UBO indexed with
resource_id, similar to the existing Object Info data. Unlike that,
however, it is necessary to maintain a separate buffer for every
requested combination of attributes.

This is done using a hash table with the attribute set as the key,
as it is not inconceivable that technically different materials may
use the same set of attributes. In addition, in order to minimize
wasted memory, a sparse UBO pool is implemented, so that chunks that
don't require that set of data don't have to allocate any memory.

Differential Revision: https://developer.blender.org/D2057

Materials: add custom object properties as uniform attributes.

This patch allows the user to type a property name into the
Attribute node, which will then output the value of the property
for each individual object, allowing to e.g. customize shaders
by object without duplicating the shader.

In order to make supporting this easier for Eevee, it is necessary
to explicitly choose whether the attribute is varying or uniform
via a dropdown option of the Attribute node. The Cycles design
seems to intend to treat all attributes equally, so the Blender
interface uses a name prefix that can't be entered with keyboard.

Differential Revision: https://developer.blender.org/D2057

Cycles: rename uchar4 attribute accessors from 'float4' to 'uchar4'.

Materials: support true float4 attributes in the Attribute node.

Add a new Alpha socket to the Attribute node that outputs the
fourth component of the attribute. Currently the only such
attribute is vertex color, but there may be more in the future.
If the attribute has no alpha channel, the expected value is 1.