[LunaMoo]

• Fixes Support for more past frames in post process effects #15668

Post shaders can ask for several previous frames with PreviousFrames=N in their ini section (plain UsePreviousFrame=True still means 1). They are bound as sampler2 (1 frame ago), sampler3 (2 frames ago) and so on, and the fragment shader gets "#define PPSSPP_PREVIOUS_FRAMES " with the count it actually got, so it can adapt with #if on devices with fewer slots.

The count is limited by what the backend can bind, keeping 2 texture slots free for future post-shader inputs (depth, multipass), and capped at 16:

• thin3d gets DeviceCaps::maxTextureSlots, MAX_TEXTURE_SLOTS goes 3 -> 20.
• GL: GL_MAX_TEXTURE_IMAGE_UNITS (GLES 2.0 minimum of 8 gives 4 frames).
• D3D11: 16 slots on FL10+, 8 on FL9.x.
• Vulkan: the thin3d descriptor set layout is sized from the device's sampler limits, and each draw pushes only the descriptors it uses.

Previous frames live in a ring of N+1 framebuffers. Until enough frames have been rendered, missing ones repeat the oldest available one, and on the very first frame the current input is bound instead.

Adds a "Motion Blur (multi-frame)" example shader.

[hrydgard]

It's kind of a cool thing to have but the use cases over 1 frame of history seem really, really limited. Not sure I want to increase the complexity here.

[LunaMoo]

Well, the motion blur can be really smooth with just few extra frames and will allow to simulate awfulness of early LCD screens ghosting, but personally I care only about having better post process effect support than RA, mostly already is, but RA has this and we don't ^_^;