gui_toolkit 0.35.2

Added

• Offscreen drawing and readback (draw/offscreen.odin, render/readback.odin, docs/draw.md): draw.offscreen_begin / offscreen_end draw into a transparent target of their own, never shown and not cut by the window; render.readback_poll gives its pixels after the frame (straight RGBA8, sRGB, alpha kept; an Op_Kind.Read copies the target where it was recorded). render.straight_srgb. Test: tests/layers offscreen_draw_and_read_back (both blend spaces, WARP).
• ui.wake_in_ms (the timed wake the last build asked for, measured from now) and ui.wake_due. wake_ms counts from the build: a loop that used it as its wait timeout and as "a frame is wanted" made a frame at every early wake of any message. Measured with Wren's tools\altdrag_sim.ps1 (AltSnap sends WM_SIZING before each step): the extra frame of each step was made for a caret blink not yet due (Ui_Timer 882 of 1544 frames of the run); with wake_due / wake_in_ms, 494 presents for 500 steps.