Fog range starts and saturates at twice the declared distance

[zoncabe]

t3d_fog_set_range doubles the offset and halves the scale. Both ends of the
range land at twice the value passed in.

tiny3d/src/t3d/t3d.c

Lines 568 to 569 in 73d822f

	float scale = 16384.0f / diff;
	float offset = -near * 2.0f;

Cause

Ucode computes (clipZ + offset) * scale and subtracts that from a full s16.
Zero fog needs clipZ == near, full fog needs the product to reach 32767. That
gives offset = -near and scale = 32768 / (far - near).

Current code halves one and doubles the other:

float scale = 16384.0f / diff;
float offset = -near * 2.0f;

Zero fog then lands at clipZ == 2*near, and saturation at 2*far.

Magma, in libdragon, runs the same computation on the same clip-space Z with
neither factor:

float factor = 1.0f / diff;
float offset = -parms->end;

Measurement

Four posts on one axis, 10 metres apart, FPS camera walking the line, both fog
distances printed on screen. Range set to 0-25 metres, fog saturates at 50.

Screenshots of both builds attached, before and after the fix. Both ROMs are
attached as well.

Fix

float scale = 32768.0f / diff;
float offset = -near;

Impact

Behaviour change, visible in every existing project. Every declared fog range
changes meaning, including the ones under examples/.

[HailToDodongo]

already fixed in PR #81