[image] New image cache

[mugulmd]

Description

This PR introduces a new image cache, associated with unit tests and a sample application.
The caching mechanism implemented is a combination of the LRU (least-recently-used) policy and reference counting with shared pointers (as we do not wish to delete images already used in some part of the application).

Features

• image cache class structure and interface
• thread-safe methods
• support all pixel types
• reload when image's last write time on disk has changed
• unit tests to cover all basic functionalities
• sample application to simulate various situations

Tests

The caching mechanism has been tested using 3 image sequences with the following characteristics:

1. dimensions: 1920x1080, nb images: 50
2. dimensions: 3424x2202, nb images: 20
3. dimensions: 3870x5796, nb images: 10

under the following situations (each several time, with varying cache memory limits, number of threads, pixel types and half-sampling levels):

• load a sequence twice
• load a sequence 6 times, each time using a different pixel type
• load all 3 sequences
• load a sequence and then reload some elements of it to check that they become MRU
• load a sequence, re-write the images on disk and reload them.

[mugulmd]

Rebased on develop and re-tested with tests in PR description

[gregoire-dl]

Tested with multiple images (read with ImageCache + write).
Tested with multiple shared pointer usages.
Tested with multiple threads.

Works fine.

I think the use of a larger type for the ImageCache capacity and for memory calculations in bytes should be done before the merge.

[mugulmd]

Just rebased on develop and force-pushed to make sure there would be no merge conflicts 😄

[fabiencastan]

Need a dedicated function to print in a human readable format, to print with MB, GB, etc depending on the values.

[fabiencastan]

precise unit

[fabiencastan]

if(it == _keys.end())
{
    break;
}

Then the main code don't need to be in a sub-scope, which is better for readability.

[fabiencastan]

Same here, inverse the condition.
The code on the top level scope should be the main one, then you deal with errors in if sub-scope.

if (memSize + _info.contentSize <= _info.maxSize)
{
    THROW...
}
load..
return _imagePtrs.at(....):

updated