[MLA-1909] Match3 and Camera/RenderTexture sensor GC improvements

[chriselion]

Proposed change(s)

• Reuse Texture2Ds to avoid allocations
• Agent now calls Dispose() on any ISensors that implement IDisposable.
• Special-case RGB24 texture->ObservationWriter (which is what Camera and RenderTexture Sensors user for inference) to avoid GetPixels32(), which allocates a copy of the pixel data.

Match3Sensor

Before

After

GridWorld (Camera + RenderTexture Sensors)

Before

After

Useful links (Github issues, JIRA tickets, ML-Agents forum threads etc.)

https://jira.unity3d.com/browse/MLA-1909
https://jira.unity3d.com/browse/MLA-1910
https://jira.unity3d.com/browse/MLA-208

Types of change(s)

• Bug fix
• New feature
• Code refactor

Checklist

• Added tests that prove my fix is effective or that my feature works
• Updated the changelog (if applicable)

Other comments

[surfnerd]

fannnncy

[surfnerd]

could this be a double dispose? looks safe since the sensor checks for null. Just curious if it's actually needed or not.

[chriselion]

I made sure all the sensor.Dispose's are idempotent - they check for null textures first, then set the texture to null.

[surfnerd]

looks good, just not sure if the extra dispose in the components are needed anymore since the sensor disposal happens in Agent.