glamor: add gl_PointSize for ES shaders

GLES3.2 spec, page 126:
> The variable gl_PointSize is intended for a shader to write
> the size of the point to be rasterized. It is measured in pixels.
> If gl_PointSize is not written to, its value
> is undefined in subsequent pipe stages.

If glamor shader is use points, we should define gl_PointSize for GLES.
On Desktop GL, it "just work" due to default gl_PointSize is 1.

As @anholt requested, define this only for minimal amount of shaders
(point and glyphbit ones), to make sure than performance will not
affected

Reviewed-by: Emma Anholt <emma@anholt.net>
Reviewed-by: Adam Jackson <ajax@redhat.com>
Signed-off-by: Konstantin <ria.freelander@gmail.com>

glamor/glamor_glyphblt.c

@@ -34,6 +34,7 @@ static const glamor_facet glamor_facet_poly_glyph_blt = {
     .name = "poly_glyph_blt",
     .vs_vars = "attribute vec2 primitive;\n",
     .vs_exec = ("       vec2 pos = vec2(0,0);\n"
+                GLAMOR_DEFAULT_POINT_SIZE
                 GLAMOR_POS(gl_Position, primitive)),
 };
 

glamor/glamor_points.c

@@ -32,7 +32,8 @@
 static const glamor_facet glamor_facet_point = {
     .name = "poly_point",
     .vs_vars = "attribute vec2 primitive;\n",
-    .vs_exec = GLAMOR_POS(gl_Position, primitive),
+    .vs_exec = (GLAMOR_DEFAULT_POINT_SIZE
+                GLAMOR_POS(gl_Position, primitive)),
 };
 
 static Bool

glamor/glamor_priv.h

@@ -49,6 +49,11 @@
     "precision mediump float;\n"  \
     "#endif\n"
 
+#define GLAMOR_DEFAULT_POINT_SIZE  \
+    "#ifdef GL_ES\n"              \
+    "       gl_PointSize = 1.0;\n"  \
+    "#endif\n"
+
 #include "glyphstr.h"
 
 #include "glamor_debug.h"