Add intermediate framebuffer for gamma correction to prevent flickering

[The-E]

Summary

cc63bfa7 ("Gamma by Shader (#7484)") introduced a shader-based gamma-correction pass that samples Back_texture (the just-rendered scene) and draws the corrected result directly into the window-system default framebuffer. This caused two SCPUI regressions, both fixed by this PR:

1. Flicker on state transitions (e.g. loading screen → briefing): on some drivers, presenting straight from a shader draw that samples a just-rendered texture isn't reliably synchronized with the swap chain, causing the screen to flicker between the outgoing and incoming screen's content for several frames.
2. Frozen briefing screen (Briefing screen (SCPUI) stops updating except while hovering a briefing icon #7590): the briefing stops visually updating (camera pan, stage text, map) except while hovering a briefing icon.

Fixes

Flicker: Route the gamma pass through an intermediate offscreen framebuffer/texture (GammaBlit_framebuffer / GammaBlit_texture), matching how every other post-process pass in the renderer works (FBO → FBO draw). The corrected result is then handed to the screen via glBlitFramebuffer, the same mechanism used for presentation before the gamma shader was introduced. This keeps gamma correction intact while removing the flicker.

Freeze: A masked draw pass inside the briefing map render leaves glColorMask fully disabled, and the Rocket UI material path never restores it, so the mask stays disabled into gr_opengl_flip. Before cc63bfa7 this was harmless — presentation was a pure glBlitFramebuffer, which ignores the color mask. The gamma pass turned presentation into a regular draw, which respects the mask, so every flip wrote nothing and the screen froze on stale swapchain content while the engine kept rendering correctly underneath. (Hovering a briefing icon appends draws whose materials re-enable color writes, which is why exactly those frames presented correctly.) The flip now neutralizes color mask and stencil state before the gamma draw, matching the blend/depth neutralization already done there. Full analysis in #7590.

Changes

• code/graphics/opengl/gropengldraw.cpp / .h: add GammaBlit_framebuffer, created alongside the existing Back_framebuffer in opengl_setup_scene_textures() and torn down in opengl_scene_texture_shutdown(). It normally reuses Scene_ldr_texture as its color attachment (dead scratch by flip time — all its consumers write before reading within gr_opengl_post_process_end()), avoiding a new screen-sized allocation; a dedicated GammaBlit_texture is only allocated in the edge case where the scene textures were clamped to GL_max_renderbuffer_size.
• code/graphics/opengl/gropengl.cpp: gr_opengl_flip() now draws the gamma-correction shader into GammaBlit_framebuffer (sampling Back_texture as before), then presents via glBlitFramebuffer into GL_BACK instead of drawing directly to the default framebuffer; it also resets color mask and stencil state before the gamma draw. When the effective gamma is 1.0 the shader pass is an identity transform and is skipped entirely, blitting Back_framebuffer straight to GL_BACK.

Testing

• Reproduced the flicker on loading screen → briefing and other state transitions before the fix; verified flicker-free transitions after, with gamma correction still visibly applied (gamma slider in options still affects brightness).
• Reproduced the briefing freeze and root-caused it with per-frame GL state/pixel probes (see Briefing screen (SCPUI) stops updating except while hovering a briefing icon #7590); verified after the fix that camera pans, stage text, and map animations update normally without hovering.
• Bisected to confirm cc63bfa7 is the sole origin of both regressions; confirmed neither occurs on its parent commit.

Fixes #7590

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