gallium: add PIPE_TEXTURE_RECT target

This allows to properly support OpenGL rectangle textures in a well
defined way, especially on drivers that don't expose
PIPE_CAP_NPOT_TEXTURES.

src/gallium/docs/source/index.rst

@@ -15,6 +15,7 @@ Contents:
    debugging
    tgsi
    screen
+   resources
    context
    cso
    distro

src/gallium/docs/source/resources.rst

@@ -0,0 +1,41 @@
+Resources
+=========
+
+Resources represent objects that hold data: textures and buffers.
+
+They are mostly modelled after the resources in Direct3D 10/11, but with a
+different transfer/update mechanism, and more features for OpenGL support.
+
+Resource targets
+----------------
+
+Resource targets determine the type of a resource.
+
+Note that drivers may not actually have the restrictions listed regarding
+coordinate normalization and wrap modes, and in fact efficient OpenCL
+support will probably require drivers that don't have any of them, which
+will probably be advertised with an appropriate cap.
+
+TODO: document all targets. Note that both 3D and cube have restrictions
+that depend on the hardware generation.
+
+TODO: can buffers have a non-R8 format?
+
+PIPE_TEXTURE_RECT
+^^^^^^^^^^^^^^^^^
+2D surface with OpenGL GL_TEXTURE_RECTANGLE semantics.
+
+depth must be 1
+- last_level must be 0
+- Must use unnormalized coordinates
+- Must use a clamp wrap mode
+
+PIPE_TEXTURE_2D
+^^^^^^^^^^^^^^^
+2D surface accessed with normalized coordinates.
+
+- If PIPE_CAP_NPOT_TEXTURES is not supported,
+      width and height must be powers of two
+- Mipmaps can be used
+- Must use normalized coordinates
+- No special restrictions on wrap modes