[feature request] pass previous frame to glsl shader

[j7432]

Add another texture output that is a copy of the last frame to a glsl shader.

(from /src/opengl.c, line 587)

// Get uniform addresses
#define P_GET_UNIFM_LOC(name, target)
{
pprogram->target = glGetUniformLocation(pprogram->prog, name);
if (pprogram->target < 0) {
log_error("Failed to get location of uniform '" name
"'. Might be troublesome.");
}
}
P_GET_UNIFM_LOC("opacity", unifm_opacity);
P_GET_UNIFM_LOC("invert_color", unifm_invert_color);
P_GET_UNIFM_LOC("tex", unifm_tex);
++ P_GET_UNIFM_LOC("prevtex", unifm_prevtex); ++
P_GET_UNIFM_LOC("time", unifm_time);
#undef P_GET_UNIFM_LOC

So that the frag shader can see the difference from previous frame (for fading,blending,blurring,etc.)

The process i am thinking of is

--frame 0--

current texture(frame0) -> shader
previous texture(null) to shader
--shader renders frame to fragcolor--
current texture(frame0) -> previous texture

--frame 1--

current texture(frame1) -> shader
previous texture(frame0) -> shader
--shader renders frame to fragcolor--
current texture(frame1) -> previous texture

etc.

I'm thinking that a framebuffer would have to be used as well.

Perhaps add a cmdline switch to only enable when needed.

If this isn't the right way to go about this, please lmk.
Thank you. j