Implement Send for Image

This changes the assumptions of the crate to assert that a valid
`pixmap::Image` has a ref-count of 1, with an alpha map that has a
ref-count of 1. Consequently, `set_alpha_map` must take ownership of its
argument.

My understanding, looking at the pixmap code, is that this should ensure
it is safe to `Send` the `Image` to another thread. Of course this does
limit the API of this crate if a consumer wants to use multiple
references (though `pixman_image_ref` wasn't wrapper already), or
wants to use `set_alpha_map` while still keeping a reference to the
alpha map. The first issue can just be solved by wrapping in `Rc`,
though.

Smithay/smithay#1500

pixman/src/image/bits.rs

@@ -13,6 +13,10 @@ pub struct Image<'bits, 'alpha> {
     _alpha: PhantomData<&'alpha ()>,
 }
 
+// SAFETY: An image is safe to `Send` if the image and its alpha map have a
+// reference count of one. So references will not exist on multiple threads.
+unsafe impl<'bits, 'alpha> Send for Image<'bits, 'alpha> {}
+
 impl<'bits, 'alpha> std::ops::Deref for Image<'bits, 'alpha> {
     type Target = ImageRef;
 
@@ -130,7 +134,7 @@ impl<'bits, 'a> Image<'bits, 'a> {
     /// used as a src in a blit operation
     pub fn set_alpha_map<'alpha: 'a>(
         self,
-        alpha_map: &'alpha Image<'_, 'static>,
+        alpha_map: Image<'alpha, 'static>,
         x: i16,
         y: i16,
     ) -> Image<'bits, 'alpha> {
@@ -467,7 +471,9 @@ impl<'bits, 'alpha> Image<'bits, 'alpha> {
     ///
     /// # Safety
     ///
-    /// The pointer is expected to be valid and have a ref-count of at least one.
+    /// The pointer is expected to be valid and have a ref-count of at *exactly* one,
+    /// with an alpha map (if any) that has a ref count of one.
+    ///
     /// Ownership of the pointer is transferred and unref will be called on drop.
     pub unsafe fn from_ptr(ptr: *mut ffi::pixman_image_t) -> Self {
         Self {

pixman/src/image/mod.rs

@@ -190,7 +190,7 @@ macro_rules! image_type {
             /// used as a src in a blit operation
             pub fn set_alpha_map<'alpha: 'a>(
                 self,
-                alpha_map: &'alpha crate::Image<'_, 'static>,
+                alpha_map: crate::Image<'alpha, 'static>,
                 x: i16,
                 y: i16,
             ) -> $name<'alpha> {