Draw shapes immediately against a renderer

BASIC 7.0's graphics verbs -- DRAW, BOX, CIRCLE, PAINT, SSHAPE and GSHAPE -- all
plot against the current screen right now rather than adding to a scene, and
draw.h had exactly one function in it. This adds akgl_draw_point, _line, _rect,
_filled_rect, _circle, _flood_fill, _copy_region and _paste_region, each taking
the akgl_RenderBackend the host already initialized.

Color is an argument rather than a current-color global, so there is no second
copy of state to disagree with the caller's own, and each call restores the
renderer's draw color when it is done. SDL3 has no circle and no flood fill: the
circle is a midpoint circle, and the fill reads the target back, walks the
region with a fixed 4096-entry span stack, and blits back only the bounding box
of what changed. Exhausting that stack reports AKERR_OUTOFBOUNDS and says in the
header that the region is left partially filled.

tests/draw.c draws into a 64x64 software renderer under the dummy video driver
and reads the pixels back, so it needs no display and no offscreen harness.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

CMakeLists.txt

@@ -157,6 +157,7 @@ set(AKGL_TEST_SUITES
   bitmasks
   character
   controller
+  draw
   error
   game
   heap