[gl] fix usage of glFlushMappedBufferRange

`offset` is relative to the start of the mapping not the start of the buffer.

wgpu-hal/src/gles/device.rs

@@ -536,6 +536,7 @@ impl crate::Device for super::Device {
                 size: desc.size,
                 map_flags: 0,
                 data: Some(Arc::new(Mutex::new(vec![0; desc.size as usize]))),
+                offset_of_current_mapping: Arc::new(Mutex::new(0)),
             });
         }
 
@@ -635,6 +636,7 @@ impl crate::Device for super::Device {
             size: desc.size,
             map_flags,
             data,
+            offset_of_current_mapping: Arc::new(Mutex::new(0)),
         })
     }
 
@@ -668,6 +670,7 @@ impl crate::Device for super::Device {
                     unsafe { self.shared.get_buffer_sub_data(gl, buffer.target, 0, slice) };
                     slice.as_mut_ptr()
                 } else {
+                    *buffer.offset_of_current_mapping.lock().unwrap() = range.start;
                     unsafe {
                         gl.map_buffer_range(
                             buffer.target,
@@ -693,6 +696,7 @@ impl crate::Device for super::Device {
                 unsafe { gl.bind_buffer(buffer.target, Some(raw)) };
                 unsafe { gl.unmap_buffer(buffer.target) };
                 unsafe { gl.bind_buffer(buffer.target, None) };
+                *buffer.offset_of_current_mapping.lock().unwrap() = 0;
             }
         }
     }
@@ -704,10 +708,11 @@ impl crate::Device for super::Device {
             let gl = &self.shared.context.lock();
             unsafe { gl.bind_buffer(buffer.target, Some(raw)) };
             for range in ranges {
+                let offset_of_current_mapping = *buffer.offset_of_current_mapping.lock().unwrap();
                 unsafe {
                     gl.flush_mapped_buffer_range(
                         buffer.target,
-                        range.start as i32,
+                        (range.start - offset_of_current_mapping) as i32,
                         (range.end - range.start) as i32,
                     )
                 };

wgpu-hal/src/gles/mod.rs

@@ -299,6 +299,7 @@ pub struct Buffer {
     size: wgt::BufferAddress,
     map_flags: u32,
     data: Option<Arc<std::sync::Mutex<Vec<u8>>>>,
+    offset_of_current_mapping: Arc<std::sync::Mutex<wgt::BufferAddress>>,
 }
 
 #[cfg(send_sync)]