blitter: Fix the sample used in the DS depth blitter shader

Since we can't properly resolve the depth buffer, we have to use one
samole, so use the sample 0 and not a random value from tc.z.

Signed-off-by: Gert Wollny <gert.wollny@collabora.com>
Reviewed-by: Corentin Noël <corentin.noel@collabora.com>.

src/vrend_blitter.h

@@ -132,14 +132,14 @@
    "uniform sampler%s samp;\n"                                           \
    "in vec4 tc;\n"                                                       \
    "void main() {\n"                                                     \
-   "   gl_FragDepth = float(texelFetch(samp, %s(tc%s), int(tc.z)).x);\n" \
+   "   gl_FragDepth = float(texelFetch(samp, %s(tc%s), 0).x);\n" \
    "}\n"
 
 #define FS_TEXFETCH_DS_MSAA_BODY_GLES                                     \
    "uniform mediump sampler%s samp;\n"                                           \
    "in vec4 tc;\n"                                                       \
    "void main() {\n"                                                     \
-   "   gl_FragDepth = float(texelFetch(samp, %s(tc%s), int(tc.z)).x);\n" \
+   "   gl_FragDepth = float(texelFetch(samp, %s(tc%s), 0).x);\n" \
    "}\n"