xgl: Add new resource type for textures

xgl: Add new resource type for textures

This change modifies xgl.h to include a new descriptor set
slot type for textures, distinct from resources:

    XGL_SLOT_SHADER_TEXTURE_RESOURCE

Buffers continue to use:

    XGL_SLOT_SHADER_RESOURCE

The ordering of their inclusion in binding tables entries is
important: textures must come before buffers.  This is how
the compiler expects them to be laid out.