tweak attrs: fix UV attribute output type.

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.

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

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

Merge branch 'blender-v2.90-release'

Eevee: do not rely on the SOCK_HIDE_VALUE flag for node group sockets.

When disconnecting links for defaulted node group inputs, recurse
into the nested node group nodes, instead of checking the socket
flag. Otherwise the behavior is confusing and differs from Cycles.

Differential Revision: https://developer.blender.org/D8455

Merge branch 'blender-v2.90-release'

Node Groups: expose the SOCK_HIDE_VALUE flag for node group inputs.

This flag specifies that even when the socket is not connected,
the node should not display the input field for the constant input
value. This is useful for inputs like Normal, which have special
handling for the missing input case and don't use a constant value.
Currently there is no way to change this flag from Python, and
through UI it can only be done by re-creating the socket.

This patch exposes the flag through RNA and UI, makes sure it
is properly updated when changed, and adds special handling to
ensure that it is correctly set when creating a node group from
a node set that includes reroute nodes.

Differential Revision: https://developer.blender.org/D8395

Node Groups: fix placement of the Output node when creating a group.

Instead of assuming the node width, compute the actual right bound.
Otherwise BSDF nodes tend to be too wide, and intersect Output.

Differential Revision: https://developer.blender.org/D8395