vrend: Use pixel pack buffers as buffers and unbind after use

On one hand it is simpler on GLES, because it allows direct readback, and
on the other hand it fixes failures when EXT_texture_buffer is not available.

Also unbind the buffers after use because otherwise glReadPixel would interpret the
pixel pointer argument as offset into the buffer and would fail.

Closes: #45

Signed-off-by: Gert Wollny <gert.wollny@collabora.com>
Acked-by: Jakob Bornecrantz <jakob@collabora.com>
Reviewed-by: Dave Airlie <airlied@redhat.com>

src/vrend_renderer.c

@@ -5560,6 +5560,7 @@ static void vrend_create_buffer(struct vrend_resource *gr, uint32_t width)
    glBindBufferARB(gr->target, gr->id);
    glBufferData(gr->target, width, NULL, GL_STREAM_DRAW);
    gr->is_buffer = true;
+   glBindBufferARB(gr->target, 0);
 }
 
 static inline void
@@ -6068,7 +6069,9 @@ static int vrend_renderer_transfer_write_iov(struct vrend_context *ctx,
        res->target == GL_ELEMENT_ARRAY_BUFFER_ARB ||
        res->target == GL_ARRAY_BUFFER_ARB ||
        res->target == GL_TEXTURE_BUFFER ||
-       res->target == GL_UNIFORM_BUFFER) {
+       res->target == GL_UNIFORM_BUFFER ||
+       res->target == GL_PIXEL_PACK_BUFFER ||
+       res->target == GL_PIXEL_UNPACK_BUFFER) {
       struct virgl_sub_upload_data d;
       d.box = info->box;
       d.target = res->target;
@@ -6082,6 +6085,7 @@ static int vrend_renderer_transfer_write_iov(struct vrend_context *ctx,
 	 vrend_read_from_iovec(iov, num_iovs, info->offset, data, info->box->width);
 	 glUnmapBuffer(res->target);
       }
+      glBindBufferARB(res->target, 0);
    } else {
       GLenum glformat;
       GLenum gltype;
@@ -6574,7 +6578,9 @@ static int vrend_renderer_transfer_send_iov(struct vrend_context *ctx,
        res->target == GL_ARRAY_BUFFER_ARB ||
        res->target == GL_TRANSFORM_FEEDBACK_BUFFER ||
        res->target == GL_TEXTURE_BUFFER ||
-       res->target == GL_UNIFORM_BUFFER) {
+       res->target == GL_UNIFORM_BUFFER ||
+       res->target == GL_PIXEL_PACK_BUFFER ||
+       res->target == GL_PIXEL_UNPACK_BUFFER) {
       uint32_t send_size = info->box->width * util_format_get_blocksize(res->base.format);
       void *data;
 
@@ -6585,6 +6591,7 @@ static int vrend_renderer_transfer_send_iov(struct vrend_context *ctx,
       else
          vrend_write_to_iovec(iov, num_iovs, info->offset, data, send_size);
       glUnmapBuffer(res->target);
+      glBindBufferARB(res->target, 0);
    } else {
       int ret = -1;
       bool can_readpixels = true;