tgsi: add properties for fragment coord conventions (v3)

Changes in v3:
- Documented the new properties
- Added comments for property values
- Rebased to current master

Changes in v2:
- Caps are added in a separate, subsequent patch

This adds two TGSI fragment program properties that indicate the
fragment coord conventions.

The properties behave as described in the extension spec for
GL_ARB_fragment_coord_conventions, but the default origin in
upper left instead of lower left as in OpenGL.

The syntax is:
PROPERTY FS_COORD_ORIGIN [UPPER_LEFT|LOWER_LEFT]
PROPERTY FS_COORD_PIXEL_CENTER [HALF_INTEGER|INTEGER]

The names have been chosen for consistency with the GS properties
and the OpenGL extension spec.

The defaults are of course the previously assumed conventions:
UPPER_LEFT and HALF_INTEGER.

src/gallium/docs/source/tgsi.rst

@@ -1319,9 +1319,8 @@ are the Cartesian coordinates, and ``w`` is the homogenous coordinate and used
 for the perspective divide, if enabled.
 
 As a vertex shader output, position should be scaled to the viewport. When
-used in fragment shaders, position will ---
-
-XXX --- wait a minute. Should position be in [0,1] for x and y?
+used in fragment shaders, position will be in window coordinates. The convention
+used depends on the FS_COORD_ORIGIN and FS_COORD_PIXEL_CENTER properties.
 
 XXX additionally, is there a way to configure the perspective divide? it's
 accelerated on most chipsets AFAIK...
@@ -1400,3 +1399,45 @@ TGSI_SEMANTIC_EDGEFLAG
 """"""""""""""""""""""
 
 XXX no clue
+
+
+Properties
+^^^^^^^^^^^^^^^^^^^^^^^^
+
+
+  Properties are general directives that apply to the whole TGSI program.
+
+FS_COORD_ORIGIN
+"""""""""""""""
+
+Specifies the fragment shader TGSI_SEMANTIC_POSITION coordinate origin.
+The default value is UPPER_LEFT.
+
+If UPPER_LEFT, the position will be (0,0) at the upper left corner and
+increase downward and rightward.
+If LOWER_LEFT, the position will be (0,0) at the lower left corner and
+increase upward and rightward.
+
+OpenGL defaults to LOWER_LEFT, and is configurable with the
+GL_ARB_fragment_coord_conventions extension.
+
+DirectX 9/10 use UPPER_LEFT.
+
+FS_COORD_PIXEL_CENTER
+"""""""""""""""""""""
+
+Specifies the fragment shader TGSI_SEMANTIC_POSITION pixel center convention.
+The default value is HALF_INTEGER.
+
+If HALF_INTEGER, the fractionary part of the position will be 0.5
+If INTEGER, the fractionary part of the position will be 0.0
+
+Note that this does not affect the set of fragments generated by
+rasterization, which is instead controlled by gl_rasterization_rules in the
+rasterizer.
+
+OpenGL defaults to HALF_INTEGER, and is configurable with the
+GL_ARB_fragment_coord_conventions extension.
+
+DirectX 9 uses INTEGER.
+DirectX 10 uses HALF_INTEGER.