"lossless texture readback" check is broken on GLES?

[sfan5]

spotted this in an mpv-android user log, so I set out to investigate:

[ 192.388][v][vo/gpu-next/libplacebo]     GL_VERSION:  OpenGL ES 3.2 V@0530.49 (GIT@329cf4c2a7, I63533b1e29, 1675175164) (Date:01/31/23)
[ 192.388][v][vo/gpu-next/libplacebo]     GL_VENDOR:   Qualcomm
[ 192.388][v][vo/gpu-next/libplacebo]     GL_RENDERER: Adreno (TM) 660
[...]
[ 192.389][e][vo/gpu-next/libplacebo] GL: read framebuffer is a framebuffer object and the selected read buffer has no image attached
[ 192.389][e][vo/gpu-next/libplacebo] GL: read framebuffer is a framebuffer object and the selected read buffer has no image attached
[ 192.389][v][vo/gpu-next/libplacebo] GPU does not seem to support lossless texture readback, restricting readback capabilities! This is a GLES/driver limitation, there is little we can do to work around it.
[ 192.389][e][vo/gpu-next/libplacebo] pl_gpu_create_gl: OpenGL error: GL_INVALID_OPERATION
[ 192.390][w][vo/gpu-next/libplacebo] Encountered errors while detecting GPU capabilities... ignoring, but expect limitations/issues

code here: https://github.com/haasn/libplacebo/blob/756220c3ee760eb62c0b89172e80f713f89fe571/src/opengl/gpu.c#L247-L271

noticed two things:

• on gles2 FramebufferTexture2D may only be called with GL_FRAMEBUFFER, but the code uses GL_DRAW_FRAMEBUFFER (their values are not identical)
• for GL_IMPLEMENTATION_COLOR_READ_* the documentation only talks about "the color buffer of the currently bound framebuffer". this makes sense on gles2 where there is only one, but on gles3 there are several. maybe it actually checks the GL_READ_FRAMEBUFFER (which is not bound), causing that error?

[sfan5]

for the second point: ES 3.2 spec, p. 409

this matches the logged error

[sfan5]

I'll open a PR for the second problem everything. As it turns out GL_[A-Z]+_FRAMEBUFFER references are all over the code and are all not compliant with GLES 2.0.