Distorted Android

Port another commit from master.

I spoke too soon, the ARM Mali flashing is of course not fixed yet. The previous commit fixed Triblur, but the bug is still reproducible elsewhere (only in the 'postprocessed' apps though).

This commit introduces a circular queue in case of the postprocessing FBOs - with little success though.

Revert last changes to the postprocessing FBO.

This reverts commit 2dbed690bdbee80d9bfb22c800073855eb5d0946.

In an attempt to contain the flashes on Mali GPU, re-structure the post-processing FBOs from 1 FBO with 2 color attachments + combined depth-stencil to 2 separate FBOs sharing a depth-stencil texture.

Result: this appears to make the flashes slightly worse.

Postprocessing: dont crash when we fail to compile postprocessing programs

Disable blending during postprocessing.

Fix for bug #28: looks of the borders of a blurred object depend on if the object is the first in the postprocessing bucket.

Postprocessing buffers mBuffer[] are now shared among all postprocessing operations. This saves a lot of memory, but also means that when doing each particular postprocessing, the textures backing up the mBuffer might be too large. We need to fix two things here: when outputting to those too large textures, we need to adjust the Viewport, and when binding those too large textures as input - we need to adjust the TexCoords to compensate....

Improve Blur (boundaries should be more white than black)

New 'Triblur' testapp.

Shows that the Blur effect doesn't fully work (probably it is the 'blitWithDepth' function which needs to be corrected)

Up library requirements to OpenGL ES 3.1 (needed for the upcoming GL_SHADER_STORAGE_BUFFER)