Depth readback with built-in stretchblit

[hrydgard]

This:

• Moves the shader-based depth convert/readback from DepthBufferGLES.cpp to DepthBufferCommon.cpp
• Makes it compatible with three of the four backends, all except D3D9 (not sure why that one doesn't work, if it did we could remove some code)
• Adds the ability to scale the image while doing the conversion. This makes it more usable and compatible than other solutions I've found, for things related to GPU readback ideas and plans #16900 .

The new stretch capability is not actually used yet, that's for upcoming PRs.