Fix an issue where the media download would not start until the end of the scroll

[nimau]

This PR fixes an issue with the back pagination where images would not download until the end of the scroll.

[codecov]

Codecov Report

Base: 37.35% // Head: 17.62% // Decreases project coverage by -19.74% ⚠️

Coverage data is based on head (10affca) compared to base (0be841e).
Patch coverage: 0.00% of modified lines in pull request are covered.

Additional details and impacted files

@@             Coverage Diff              @@
##           develop    #1721       +/-   ##
============================================
- Coverage    37.35%   17.62%   -19.74%     
============================================
  Files          612      612               
  Lines        96133    96132        -1     
  Branches     41556    40348     -1208     
============================================
- Hits         35912    16939    -18973     
- Misses       59171    78683    +19512     
+ Partials      1050      510      -540     

Impacted Files	Coverage Δ
MatrixSDK/Utils/Media/MXMediaLoader.m	0.00% <0.00%> (ø)
MatrixSDKTests/MatrixSDKTestsData.h	0.00% <0.00%> (-100.00%)	⬇️
MatrixSDK/Utils/MXDateProvider.swift	0.00% <0.00%> (-100.00%)	⬇️
MatrixSDK/Crypto/Utils/MXCryptoTools.m	0.00% <0.00%> (-100.00%)	⬇️
MatrixSDK/Crypto/Devices/MXDeviceList.h	0.00% <0.00%> (-100.00%)	⬇️
MatrixSDK/Crypto/Data/MXOlmSessionResult.m	0.00% <0.00%> (-100.00%)	⬇️
MatrixSDK/Crypto/Migration/MXCryptoVersion.h	0.00% <0.00%> (-100.00%)	⬇️
MatrixSDK/JSONModels/Sync/Room/MXRoomSyncState.m	0.00% <0.00%> (-100.00%)	⬇️
MatrixSDK/JSONModels/Sync/MXPresenceSyncResponse.m	0.00% <0.00%> (-100.00%)	⬇️
...atrixSDK/JSONModels/Sync/Room/MXRoomSyncTimeline.m	0.00% <0.00%> (-100.00%)	⬇️
... and 187 more

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[stefanceriu]

👍