Add support for runtime mipmap generation.

[mtwilliams]

Runtime mipmap generation should be exposed for texture objects.

The cleanest approach would be to provide a stateless method that explicitly generates mipmaps from the top-most level, i.e., agl_texture_generate_mipmaps(). However, this violates the zero-copy design philosophy of AGL, and is impossible for immutable texture objects. Thus, agl_texture_data_1d/2d/3d() should accept flags, of which AUTO_GENERATE_MIPMAPS should implement the desired behavior.

agl_texture_data_2d(texture, x, y, w, h, format, data, AGL_TEXTURE_AUTO_GENERATE_MIPMAPS);

((agl::Texture2D *)texture)->data(x, y, w, h, format, data, agl::Texture::AutoGenerateMipmaps);

Alternatively, a boolean texture parameter can be exposed that results in similar behavior:

agl_texture_set_parameter_boolean(texture, AGL_TEXTURE_PARAMETER_AUTO_GENERATE_MIPMAPS, 1);
agl_texture_data_2d(texture, x, y, w, h, format, data);

texture->set_parameter(agl::Texture::Parameters::AutoGenerateMipmaps, true);
((agl::Texture2D *)texture)->data(x, y, w, h, format, data);

[mtwilliams]

For Direct3D9, if IDirect3DBaseTexture9::GetAutoGenFilterType returns D3DTEXF_NONE then calls to IDirect3DBaseTexture9::GetLevelCount will return incorrect values, thus affecting AGL_TEXTURE_PARAMETER_LEVEL_OF_DETAIL, see #3

To generate sub-levels texture objects need to:

1. Not be in the system memory pool, D3DPOOL_SYSTEMMEM; and
2. have D3DUSAGE_AUTOGENMIPMAP specified.

Then calls to IDirect3DBaseTexture9::GenerateMipSubLevels can be used to hint when sublevels should be generated, i.e. in agl_texture_data_1d/2d/3d.

[mtwilliams]

For Direct3D11, calls to ID3D11DeviceContext::GenerateMips for non-render-target views have no effect, thus auto-generation must implemented by AGL.

[mtwilliams]

For OpenGL 2.1+, and OpenGL 3.3+, calls to glGenerateMipmapEXT should be preceded with glEnable calls, as a work around ATI/AMD driver bugs. [1]

Additionally, as a fallback, set the texture parameter GL_GENERATE_MIPMAP to GL_TRUE then specify texture data and then rebind the texture. This however results in the implicit generation of mipmaps, i.e., it hints to the driver to generate sublevels. Furthermore, it is a Pandora's box of issues especially with regards to framebuffers and unbatched textures updates. [2]

[mtwilliams]

For OpenGL ES 2.0 and OpenGL ES 3.0, just use glGenerateMipmap.

[mtwilliams]

Finally, for other backends (presumably including mantle), auto-generation must be implemented by AGL.