mesa: fix _mesa_texstore_argb8888() for MESA_FORMAT_XRGB8888

If we hit the general path and call _mesa_make_temp_chan_image() we
always want to get a GL_RGBA texture.  We were getting a 3-channel
GL_RGB texture before and that messed up the memory layout.

src/mesa/main/texstore.c

@@ -1406,7 +1406,7 @@ _mesa_texstore_argb8888(TEXSTORE_PARAMS)
 {
    const GLboolean littleEndian = _mesa_little_endian();
    const GLuint texelBytes = _mesa_get_format_bytes(dstFormat);
-   const GLenum baseFormat = _mesa_get_format_base_format(dstFormat);
+   const GLenum baseFormat = GL_RGBA;
 
    ASSERT(dstFormat == MESA_FORMAT_ARGB8888 ||
           dstFormat == MESA_FORMAT_ARGB8888_REV ||