Allow upscaling of PCX images using HQ4x filter.

Upscale 4x when ‘gl_upscale_pcx’ is set to 2.

Remove unused rules from HQ2x blending routine, reorder existing rules
for compatibility with HQ4x. Rename rules to make their weights obvious.

Optimize HQx initialization routine, skip entirely when ‘gl_upscale_pcx’
is set to 0.

Add ‘hqx_y’, ‘hqx_cb’ and ‘hqx_cr’ console variables for fine-tuning the
HQx algorithm.

Avoid performance bottleneck when scrap texture is uploaded several
times in a row. Delay scrap uploading until the next drawing call.