fix(gles): use STREAM_DRAW for PIXEL_PACK_BUFFER (fixes #1595)

[xdagiz]

Description

The GLES renderer was using GL_STREAM_READ when allocating PBOs for
glReadPixels operations (i don't know if this was a bug or intentional). However, PIXEL_PACK_BUFFER is used to pack
pixel data FROM the GPU (GPU writes, CPU reads), so using GL_STREAM_DRAW fixed it.

This caused screenshots in niri to fail with "failed to copy to output" and with debug logs:

ERROR smithay::backend::renderer::gles: [GL] GL_INVALID_ENUM in glBufferData(invalid usage: GL_STREAM_READ)
ERROR smithay::backend::renderer::gles: [GL] GL_INVALID_OPERATION in glReadPixels(out of bounds PBO access)

on older CPUs (particularly Intel integrated graphics), where drivers
enforce the usage hint more strictly.

Checklist

• I agree to smithay's Developer Certificate of Origin.

[iggylari]

Works with Niri on i7 L620, thanks

[xdagiz]

@iggylari yeah it also works on my old intel core i2

[Drakulix]

Thanks!

[YaLTeR]

Confirming it fixes screenshots on niri on my Eee PC