fix(graphics): don't require CONTEXT block on every progressive frame

Per MS-RDPEGFX 2.2.4.2 the SYNC + CONTEXT blocks establish a codec
context once (keyed by codec_context_id) and are not repeated on
subsequent frames that reference the same context. The decoder's strict
requirement that CONTEXT be present on every decode_bitmap call rejected
every frame after the first with MissingBlock("CONTEXT"), freezing the
image on the coarse first pass.

Real-world servers (xrdp, GNOME Remote Desktop) omit the CONTEXT block
on every frame after the first one that established the context. Fall
back to the value stored when the context was first created; only error
when the very first frame for a context arrives without CONTEXT.

Author: rdeangel

crates/ironrdp-graphics/src/progressive.rs

@@ -1076,16 +1076,30 @@ impl ProgressiveDecoder {
 
         let blocks = decode_progressive_stream(bitmap_data)?;
 
-        // Extract context flags from the CONTEXT block. Per MS-RDPEGFX 2.2.4.2
-        // a Progressive stream MUST begin with SYNC + CONTEXT; treat absence as
-        // a malformed stream rather than silently defaulting band layout.
-        let use_reduce_extrapolate = blocks
-            .iter()
-            .find_map(|block| match block {
-                ProgressiveBlock::Context(ctx) => Some(ctx.uses_reduce_extrapolate()),
-                _ => None,
-            })
-            .ok_or(ProgressiveDecodeError::MissingBlock("CONTEXT"))?;
+        // Extract the band-layout flag from the CONTEXT block when present.
+        // Per MS-RDPEGFX 2.2.4.2 the SYNC + CONTEXT blocks establish a codec
+        // context once (keyed by `codec_context_id`) and are not required to be
+        // repeated on subsequent frames that reference the same context.
+        // Real-world servers (xrdp, GNOME Remote Desktop) omit the CONTEXT
+        // block on every frame after the first one that established the
+        // context. The strict requirement rejected each of those frames with
+        // `MissingBlock("CONTEXT")`, freezing the image on the coarse first
+        // pass.
+        //
+        // Fall back to the value stored when the context was first created.
+        // Only error when neither source is available, i.e. the very first
+        // frame for a context arrived without a CONTEXT block.
+        let use_reduce_extrapolate = match blocks.iter().find_map(|block| match block {
+            ProgressiveBlock::Context(ctx) => Some(ctx.uses_reduce_extrapolate()),
+            _ => None,
+        }) {
+            Some(v) => v,
+            None => self
+                .contexts
+                .get(&codec_context_id)
+                .map(|c| c.surface.use_reduce_extrapolate)
+                .ok_or(ProgressiveDecodeError::MissingBlock("CONTEXT"))?,
+        };
 
         // Get or create the context for this codec_context_id
         let context = match self.contexts.entry(codec_context_id) {