vrend: fix glTexParameteri call

When emulating GL_ALPHA8 with GL_R8, we should make the alpha
component equal to the red color channel.

Fixes:
['dEQP-GLES2.functional.texture.mipmap.2d.generate.a8_fastest']
['dEQP-GLES2.functional.texture.mipmap.2d.generate.a8_nicest']

on OpenGL 4.5 (core).
Tested-by: Jakob Bornecrantz <jakob@collabora.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>

src/vrend_blitter.c

@@ -616,7 +616,7 @@ void vrend_renderer_blit_gl(struct vrend_context *ctx,
    glBindTexture(src_res->target, src_res->id);
 
    if (vrend_format_is_emulated_alpha(info->src.format))
-      glTexParameteri(src_res->target, GL_TEXTURE_SWIZZLE_R, GL_ALPHA);
+      glTexParameteri(src_res->target, GL_TEXTURE_SWIZZLE_A, GL_RED);
 
    glTexParameteri(src_res->target, GL_TEXTURE_WRAP_S, GL_CLAMP_TO_EDGE);
    glTexParameteri(src_res->target, GL_TEXTURE_WRAP_T, GL_CLAMP_TO_EDGE);