Removed beta notice from README.md

Added `vram_texture_preload_size` configuration option

Implemented naive speculative loading of material images. At startup, if
the `vram_texture_preload_size` option is specified, a random collection
of materials is preloaded into video memory.

At low values, `vram_texture_preload_size` will reduce (but not
eliminate) run-time stuttering due to texture streaming. However, if
this value is arbitrarily large, and there is sufficient video memory
available, all material images can be preloaded at program launch.

There is significant potential future work in this area, but for now I
am considering the speculative loading issue fixed. More tasks can be
opened to track alternative approaches in the future, as needed.

Fixes jdmclark#6

Amortize HUD texture update over 4 frames

Fixes jdmclark#63

Removed use of extension `GL_ARB_explicit_uniform_location`

This change eliminates the last OpenGL extension in use,
`GL_ARB_explicit_uniform_location`. This change should improve
compatibility with certain hardware and driver combinations.

Fixes jdmclark#54

Performance improvement for SSAO

Previously, depth and normal values were packed into a single texture.
This change separates depth and normal into different textures, greatly
reducing the cost of random depth lookup inside the SSAO shader.

Performance impact is hardware- and resolution-dependent. Local testing
suggests this improvement is substantial (GTX 1080 @ 4k saw framerates
increase from roughly 30 to roughly 58 with SSAO enabled).

Update release.yml

Updated release workflow to run release when pushing to a version tag

Replaced the `internal_resolution` setting

Previously, jkgfxmod provided an `internal_resolution` feature. This
optional feature would render the game at a different internal
resolution than the native screen resolution. The intended use for this
feature was to pillarbox widescreen resolutions, but it was implemented
in a manner that is much too generic and difficult to explain.

The `internal_resolution` feature has been replaced with a simple
`correct_game_aspect_ratio` toggle, matching the previously-implemented
`correct_menu_aspect_ratio` option. When `correct_game_aspect_ratio` is
enabled, the game renders at an internal 4:3 aspect ratio which best
fits the configured screen resolution.

Existing configuration files will work, but the `internal_resolution`
field will be ignored.

Fixed post_box4.frag shader compile error on some graphics hardware

The previous version of post_box4.frag unintentionally used a
proprietary extension, and did not work with compliant GLSL compilers.
This has been corrected.

Fixes jdmclark#29

Update README.md

Purely symbolic promotion from alpha to beta, due to expanded testing.

Added an option to enable or disable bloom

This configuration option is named "enable_bloom", and defaults to true.