shader: emit precision for images on GLSL

When using OpenGL ES 3.1, we either need to emit a precision qualifier,
or set a default precision.

Since it's not unthinkable that we'd want to forward lowp/mediump
in the future, let's use the precision qualifier so it's easier
to get this right later on.

This gets rid of a shader compile error.

Signed-off-by: Erik Faye-Lund <erik.faye-lund@collabora.com>
Reviewed-by: Gurchetan Singh <gurchetansingh@chromium.org>
Signed-off-by: Dave Airlie <airlied@redhat.com>

src/vrend_shader.c

@@ -4284,6 +4284,7 @@ static void *emit_image_decl(const struct dump_ctx *ctx, char *glsl_hdr,
    const char *sname, *stc, *formatstr;
    enum tgsi_return_type itype;
    const char *volatile_str = image->vflag ? "volatile " : "";
+   const char *precision = ctx->cfg->use_gles ? "highp " : "";
    const char *access = "";
    formatstr = get_internalformat_string(image->decl.Format, &itype);
    ptc = vrend_shader_samplerreturnconv(itype);
@@ -4305,11 +4306,11 @@ static void *emit_image_decl(const struct dump_ctx *ctx, char *glsl_hdr,
    }
 
    if (range)
-      snprintf(buf, 255, "%s%suniform %cimage%s %simg%d[%d];\n",
+      snprintf(buf, 255, "%s%suniform %s%cimage%s %simg%d[%d];\n",
-               access, volatile_str, ptc, stc, sname, i, range);
+               access, volatile_str, precision, ptc, stc, sname, i, range);
    else
-      snprintf(buf, 255, "%s%suniform %cimage%s %simg%d;\n",
+      snprintf(buf, 255, "%s%suniform %s%cimage%s %simg%d;\n",
-               access, volatile_str, ptc, stc, sname, i);
+               access, volatile_str, precision, ptc, stc, sname, i);
 
    STRCAT_WITH_RET(glsl_hdr, buf);
    return glsl_hdr;