imgui 1.92 hands the backend a list of dirty rectangles when its font atlas
grows, but thin3d could only replace a whole mip level, so every new glyph
re-uploaded the entire atlas.
Adds DrawContext::UpdateTextureRegions, taking a batch of regions so each
backend can submit them together:
- Vulkan: packs the regions into the push pool and issues a single
vkCmdCopyBufferToImage from the init command buffer, transitioning only
the level being written.
- OpenGL: new TEXTURE_SUBIMAGE init step. The existing sub-image path is a
render command needing an active render pass and a texture slot, which
doesn't fit here. Rows are packed caller-side since GLES2 lacks
GL_UNPACK_ROW_LENGTH.
- D3D11: UpdateSubresource with a box, no staging texture needed.
Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
